Merge branch 'writeback-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux

* 'writeback-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux:
  writeback: Add a 'reason' to wb_writeback_work
  writeback: send work item to queue_io, move_expired_inodes
  writeback: trace event balance_dirty_pages
  writeback: trace event bdi_dirty_ratelimit
  writeback: fix ppc compile warnings on do_div(long long, unsigned long)
  writeback: per-bdi background threshold
  writeback: dirty position control - bdi reserve area
  writeback: control dirty pause time
  writeback: limit max dirty pause time
  writeback: IO-less balance_dirty_pages()
  writeback: per task dirty rate limit
  writeback: stabilize bdi->dirty_ratelimit
  writeback: dirty rate control
  writeback: add bg_threshold parameter to __bdi_update_bandwidth()
  writeback: dirty position control
  writeback: account per-bdi accumulated dirtied pages

3 files changed, 560 insertions, 154 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7520ef0bfd47..a0860640378d 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -97,6 +97,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
                    "BdiDirtyThresh:     %10lu kB\n"
                    "DirtyThresh:        %10lu kB\n"
                    "BackgroundThresh:   %10lu kB\n"
+                   "BdiDirtied:         %10lu kB\n"
                    "BdiWritten:         %10lu kB\n"
                    "BdiWriteBandwidth:  %10lu kBps\n"
                    "b_dirty:            %10lu\n"
@@ -109,6 +110,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
                    K(bdi_thresh),
                    K(dirty_thresh),
                    K(background_thresh),
+                   (unsigned long) K(bdi_stat(bdi, BDI_DIRTIED)),
                    (unsigned long) K(bdi_stat(bdi, BDI_WRITTEN)),
                    (unsigned long) K(bdi->write_bandwidth),
                    nr_dirty,
@@ -473,7 +475,8 @@ static int bdi_forker_thread(void *ptr)
                                  * the bdi from the thread. Hopefully 1024 is
                                  * large enough for efficient IO.
                                  */
-                                writeback_inodes_wb(&bdi->wb, 1024);
+                                writeback_inodes_wb(&bdi->wb, 1024,
+                                                    WB_REASON_FORKER_THREAD);
                         } else {
                                 /*
                                  * The spinlock makes sure we do not lose
@@ -683,6 +686,8 @@ int bdi_init(struct backing_dev_info *bdi)
         bdi->bw_time_stamp = jiffies;
         bdi->written_stamp = 0;
 
+        bdi->balanced_dirty_ratelimit = INIT_BW;
+        bdi->dirty_ratelimit = INIT_BW;
         bdi->write_bandwidth = INIT_BW;
         bdi->avg_write_bandwidth = INIT_BW;
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 793e9874de51..9c31199dae9a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -46,26 +46,14 @@
  */
 #define BANDWIDTH_INTERVAL      max(HZ/5, 1)
 
+#define RATELIMIT_CALC_SHIFT    10
+
 /*
  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
  * will look to see if it needs to force writeback or throttling.
  */
 static long ratelimit_pages = 32;
 
-/*
- * When balance_dirty_pages decides that the caller needs to perform some
- * non-background writeback, this is how many pages it will attempt to write.
- * It should be somewhat larger than dirtied pages to ensure that reasonably
- * large amounts of I/O are submitted.
- */
-static inline long sync_writeback_pages(unsigned long dirtied)
-{
-        if (dirtied < ratelimit_pages)
-                dirtied = ratelimit_pages;
-
-        return dirtied + dirtied / 2;
-}
-
 /* The following parameters are exported via /proc/sys/vm */
 
 /*
@@ -167,6 +155,8 @@ static void update_completion_period(void)
         int shift = calc_period_shift();
         prop_change_shift(&vm_completions, shift);
         prop_change_shift(&vm_dirties, shift);
+
+        writeback_set_ratelimit();
 }
 
 int dirty_background_ratio_handler(struct ctl_table *table, int write,
@@ -260,50 +250,6 @@ static void bdi_writeout_fraction(struct backing_dev_info *bdi,
                                 numerator, denominator);
 }
 
-static inline void task_dirties_fraction(struct task_struct *tsk,
-                long *numerator, long *denominator)
-{
-        prop_fraction_single(&vm_dirties, &tsk->dirties,
-                                numerator, denominator);
-}
-
-/*
- * task_dirty_limit - scale down dirty throttling threshold for one task
- *
- * task specific dirty limit:
- *
- *   dirty -= (dirty/8) * p_{t}
- *
- * To protect light/slow dirtying tasks from heavier/fast ones, we start
- * throttling individual tasks before reaching the bdi dirty limit.
- * Relatively low thresholds will be allocated to heavy dirtiers. So when
- * dirty pages grow large, heavy dirtiers will be throttled first, which will
- * effectively curb the growth of dirty pages. Light dirtiers with high enough
- * dirty threshold may never get throttled.
- */
-#define TASK_LIMIT_FRACTION 8
-static unsigned long task_dirty_limit(struct task_struct *tsk,
-                                       unsigned long bdi_dirty)
-{
-        long numerator, denominator;
-        unsigned long dirty = bdi_dirty;
-        u64 inv = dirty / TASK_LIMIT_FRACTION;
-
-        task_dirties_fraction(tsk, &numerator, &denominator);
-        inv *= numerator;
-        do_div(inv, denominator);
-
-        dirty -= inv;
-
-        return max(dirty, bdi_dirty/2);
-}
-
-/* Minimum limit for any task */
-static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
-{
-        return bdi_dirty - bdi_dirty / TASK_LIMIT_FRACTION;
-}
-
 /*
  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
  * registered backing devices, which, for obvious reasons, can not
@@ -413,6 +359,12 @@ unsigned long determine_dirtyable_memory(void)
         return x + 1;   /* Ensure that we never return 0 */
 }
 
+static unsigned long dirty_freerun_ceiling(unsigned long thresh,
+                                           unsigned long bg_thresh)
+{
+        return (thresh + bg_thresh) / 2;
+}
+
 static unsigned long hard_dirty_limit(unsigned long thresh)
 {
         return max(thresh, global_dirty_limit);
@@ -497,6 +449,198 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
         return bdi_dirty;
 }
 
+/*
+ * Dirty position control.
+ *
+ * (o) global/bdi setpoints
+ *
+ * We want the dirty pages be balanced around the global/bdi setpoints.
+ * When the number of dirty pages is higher/lower than the setpoint, the
+ * dirty position control ratio (and hence task dirty ratelimit) will be
+ * decreased/increased to bring the dirty pages back to the setpoint.
+ *
+ *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
+ *
+ *     if (dirty < setpoint) scale up   pos_ratio
+ *     if (dirty > setpoint) scale down pos_ratio
+ *
+ *     if (bdi_dirty < bdi_setpoint) scale up   pos_ratio
+ *     if (bdi_dirty > bdi_setpoint) scale down pos_ratio
+ *
+ *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
+ *
+ * (o) global control line
+ *
+ *     ^ pos_ratio
+ *     |
+ *     |            |<===== global dirty control scope ======>|
+ * 2.0 .............*
+ *     |            .*
+ *     |            . *
+ *     |            .   *
+ *     |            .     *
+ *     |            .        *
+ *     |            .            *
+ * 1.0 ................................*
+ *     |            .                  .     *
+ *     |            .                  .          *
+ *     |            .                  .              *
+ *     |            .                  .                 *
+ *     |            .                  .                    *
+ *   0 +------------.------------------.----------------------*------------->
+ *           freerun^          setpoint^                 limit^   dirty pages
+ *
+ * (o) bdi control line
+ *
+ *     ^ pos_ratio
+ *     |
+ *     |            *
+ *     |              *
+ *     |                *
+ *     |                  *
+ *     |                    * |<=========== span ============>|
+ * 1.0 .......................*
+ *     |                      . *
+ *     |                      .   *
+ *     |                      .     *
+ *     |                      .       *
+ *     |                      .         *
+ *     |                      .           *
+ *     |                      .             *
+ *     |                      .               *
+ *     |                      .                 *
+ *     |                      .                   *
+ *     |                      .                     *
+ * 1/4 ...............................................* * * * * * * * * * * *
+ *     |                      .                         .
+ *     |                      .                           .
+ *     |                      .                             .
+ *   0 +----------------------.-------------------------------.------------->
+ *                bdi_setpoint^                    x_intercept^
+ *
+ * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can
+ * be smoothly throttled down to normal if it starts high in situations like
+ * - start writing to a slow SD card and a fast disk at the same time. The SD
+ *   card's bdi_dirty may rush to many times higher than bdi_setpoint.
+ * - the bdi dirty thresh drops quickly due to change of JBOD workload
+ */
+static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
+                                        unsigned long thresh,
+                                        unsigned long bg_thresh,
+                                        unsigned long dirty,
+                                        unsigned long bdi_thresh,
+                                        unsigned long bdi_dirty)
+{
+        unsigned long write_bw = bdi->avg_write_bandwidth;
+        unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
+        unsigned long limit = hard_dirty_limit(thresh);
+        unsigned long x_intercept;
+        unsigned long setpoint;         /* dirty pages' target balance point */
+        unsigned long bdi_setpoint;
+        unsigned long span;
+        long long pos_ratio;            /* for scaling up/down the rate limit */
+        long x;
+
+        if (unlikely(dirty >= limit))
+                return 0;
+
+        /*
+         * global setpoint
+         *
+         *                           setpoint - dirty 3
+         *        f(dirty) := 1.0 + (----------------)
+         *                           limit - setpoint
+         *
+         * it's a 3rd order polynomial that subjects to
+         *
+         * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
+         * (2) f(setpoint) = 1.0 => the balance point
+         * (3) f(limit)    = 0   => the hard limit
+         * (4) df/dx      <= 0   => negative feedback control
+         * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
+         *     => fast response on large errors; small oscillation near setpoint
+         */
+        setpoint = (freerun + limit) / 2;
+        x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
+                    limit - setpoint + 1);
+        pos_ratio = x;
+        pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+        pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+        pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
+
+        /*
+         * We have computed basic pos_ratio above based on global situation. If
+         * the bdi is over/under its share of dirty pages, we want to scale
+         * pos_ratio further down/up. That is done by the following mechanism.
+         */
+
+        /*
+         * bdi setpoint
+         *
+         *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
+         *
+         *                        x_intercept - bdi_dirty
+         *                     := --------------------------
+         *                        x_intercept - bdi_setpoint
+         *
+         * The main bdi control line is a linear function that subjects to
+         *
+         * (1) f(bdi_setpoint) = 1.0
+         * (2) k = - 1 / (8 * write_bw)  (in single bdi case)
+         *     or equally: x_intercept = bdi_setpoint + 8 * write_bw
+         *
+         * For single bdi case, the dirty pages are observed to fluctuate
+         * regularly within range
+         *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
+         * for various filesystems, where (2) can yield in a reasonable 12.5%
+         * fluctuation range for pos_ratio.
+         *
+         * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its
+         * own size, so move the slope over accordingly and choose a slope that
+         * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.
+         */
+        if (unlikely(bdi_thresh > thresh))
+                bdi_thresh = thresh;
+        bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
+        /*
+         * scale global setpoint to bdi's:
+         *      bdi_setpoint = setpoint * bdi_thresh / thresh
+         */
+        x = div_u64((u64)bdi_thresh << 16, thresh + 1);
+        bdi_setpoint = setpoint * (u64)x >> 16;
+        /*
+         * Use span=(8*write_bw) in single bdi case as indicated by
+         * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
+         *
+         *        bdi_thresh                    thresh - bdi_thresh
+         * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
+         *          thresh                            thresh
+         */
+        span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
+        x_intercept = bdi_setpoint + span;
+
+        if (bdi_dirty < x_intercept - span / 4) {
+                pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
+                                    x_intercept - bdi_setpoint + 1);
+        } else
+                pos_ratio /= 4;
+
+        /*
+         * bdi reserve area, safeguard against dirty pool underrun and disk idle
+         * It may push the desired control point of global dirty pages higher
+         * than setpoint.
+         */
+        x_intercept = bdi_thresh / 2;
+        if (bdi_dirty < x_intercept) {
+                if (bdi_dirty > x_intercept / 8)
+                        pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);
+                else
+                        pos_ratio *= 8;
+        }
+
+        return pos_ratio;
+}
+
 static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
                                        unsigned long elapsed,
                                        unsigned long written)
@@ -593,8 +737,153 @@ static void global_update_bandwidth(unsigned long thresh,
         spin_unlock(&dirty_lock);
 }
 
+/*
+ * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
+ *
+ * Normal bdi tasks will be curbed at or below it in long term.
+ * Obviously it should be around (write_bw / N) when there are N dd tasks.
+ */
+static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
+                                       unsigned long thresh,
+                                       unsigned long bg_thresh,
+                                       unsigned long dirty,
+                                       unsigned long bdi_thresh,
+                                       unsigned long bdi_dirty,
+                                       unsigned long dirtied,
+                                       unsigned long elapsed)
+{
+        unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
+        unsigned long limit = hard_dirty_limit(thresh);
+        unsigned long setpoint = (freerun + limit) / 2;
+        unsigned long write_bw = bdi->avg_write_bandwidth;
+        unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
+        unsigned long dirty_rate;
+        unsigned long task_ratelimit;
+        unsigned long balanced_dirty_ratelimit;
+        unsigned long pos_ratio;
+        unsigned long step;
+        unsigned long x;
+
+        /*
+         * The dirty rate will match the writeout rate in long term, except
+         * when dirty pages are truncated by userspace or re-dirtied by FS.
+         */
+        dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed;
+
+        pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
+                                       bdi_thresh, bdi_dirty);
+        /*
+         * task_ratelimit reflects each dd's dirty rate for the past 200ms.
+         */
+        task_ratelimit = (u64)dirty_ratelimit *
+                                        pos_ratio >> RATELIMIT_CALC_SHIFT;
+        task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
+
+        /*
+         * A linear estimation of the "balanced" throttle rate. The theory is,
+         * if there are N dd tasks, each throttled at task_ratelimit, the bdi's
+         * dirty_rate will be measured to be (N * task_ratelimit). So the below
+         * formula will yield the balanced rate limit (write_bw / N).
+         *
+         * Note that the expanded form is not a pure rate feedback:
+         *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate)              (1)
+         * but also takes pos_ratio into account:
+         *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
+         *
+         * (1) is not realistic because pos_ratio also takes part in balancing
+         * the dirty rate.  Consider the state
+         *      pos_ratio = 0.5                                              (3)
+         *      rate = 2 * (write_bw / N)                                    (4)
+         * If (1) is used, it will stuck in that state! Because each dd will
+         * be throttled at
+         *      task_ratelimit = pos_ratio * rate = (write_bw / N)           (5)
+         * yielding
+         *      dirty_rate = N * task_ratelimit = write_bw                   (6)
+         * put (6) into (1) we get
+         *      rate_(i+1) = rate_(i)                                        (7)
+         *
+         * So we end up using (2) to always keep
+         *      rate_(i+1) ~= (write_bw / N)                                 (8)
+         * regardless of the value of pos_ratio. As long as (8) is satisfied,
+         * pos_ratio is able to drive itself to 1.0, which is not only where
+         * the dirty count meet the setpoint, but also where the slope of
+         * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
+         */
+        balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
+                                           dirty_rate | 1);
+
+        /*
+         * We could safely do this and return immediately:
+         *
+         *      bdi->dirty_ratelimit = balanced_dirty_ratelimit;
+         *
+         * However to get a more stable dirty_ratelimit, the below elaborated
+         * code makes use of task_ratelimit to filter out sigular points and
+         * limit the step size.
+         *
+         * The below code essentially only uses the relative value of
+         *
+         *      task_ratelimit - dirty_ratelimit
+         *      = (pos_ratio - 1) * dirty_ratelimit
+         *
+         * which reflects the direction and size of dirty position error.
+         */
+
+        /*
+         * dirty_ratelimit will follow balanced_dirty_ratelimit iff
+         * task_ratelimit is on the same side of dirty_ratelimit, too.
+         * For example, when
+         * - dirty_ratelimit > balanced_dirty_ratelimit
+         * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
+         * lowering dirty_ratelimit will help meet both the position and rate
+         * control targets. Otherwise, don't update dirty_ratelimit if it will
+         * only help meet the rate target. After all, what the users ultimately
+         * feel and care are stable dirty rate and small position error.
+         *
+         * |task_ratelimit - dirty_ratelimit| is used to limit the step size
+         * and filter out the sigular points of balanced_dirty_ratelimit. Which
+         * keeps jumping around randomly and can even leap far away at times
+         * due to the small 200ms estimation period of dirty_rate (we want to
+         * keep that period small to reduce time lags).
+         */
+        step = 0;
+        if (dirty < setpoint) {
+                x = min(bdi->balanced_dirty_ratelimit,
+                         min(balanced_dirty_ratelimit, task_ratelimit));
+                if (dirty_ratelimit < x)
+                        step = x - dirty_ratelimit;
+        } else {
+                x = max(bdi->balanced_dirty_ratelimit,
+                         max(balanced_dirty_ratelimit, task_ratelimit));
+                if (dirty_ratelimit > x)
+                        step = dirty_ratelimit - x;
+        }
+
+        /*
+         * Don't pursue 100% rate matching. It's impossible since the balanced
+         * rate itself is constantly fluctuating. So decrease the track speed
+         * when it gets close to the target. Helps eliminate pointless tremors.
+         */
+        step >>= dirty_ratelimit / (2 * step + 1);
+        /*
+         * Limit the tracking speed to avoid overshooting.
+         */
+        step = (step + 7) / 8;
+
+        if (dirty_ratelimit < balanced_dirty_ratelimit)
+                dirty_ratelimit += step;
+        else
+                dirty_ratelimit -= step;
+
+        bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
+        bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
+
+        trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
+}
+
 void __bdi_update_bandwidth(struct backing_dev_info *bdi,
                             unsigned long thresh,
+                            unsigned long bg_thresh,
                             unsigned long dirty,
                             unsigned long bdi_thresh,
                             unsigned long bdi_dirty,
@@ -602,6 +891,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
 {
         unsigned long now = jiffies;
         unsigned long elapsed = now - bdi->bw_time_stamp;
+        unsigned long dirtied;
         unsigned long written;
 
         /*
@@ -610,6 +900,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
         if (elapsed < BANDWIDTH_INTERVAL)
                 return;
 
+        dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
         written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
 
         /*
@@ -619,18 +910,23 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
         if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
                 goto snapshot;
 
-        if (thresh)
+        if (thresh) {
                 global_update_bandwidth(thresh, dirty, now);
-
+                bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
+                                           bdi_thresh, bdi_dirty,
+                                           dirtied, elapsed);
+        }
         bdi_update_write_bandwidth(bdi, elapsed, written);
 
 snapshot:
+        bdi->dirtied_stamp = dirtied;
         bdi->written_stamp = written;
         bdi->bw_time_stamp = now;
 }
 
 static void bdi_update_bandwidth(struct backing_dev_info *bdi,
                                  unsigned long thresh,
+                                 unsigned long bg_thresh,
                                  unsigned long dirty,
                                  unsigned long bdi_thresh,
                                  unsigned long bdi_dirty,
@@ -639,37 +935,99 @@ static void bdi_update_bandwidth(struct backing_dev_info *bdi,
         if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
                 return;
         spin_lock(&bdi->wb.list_lock);
-        __bdi_update_bandwidth(bdi, thresh, dirty, bdi_thresh, bdi_dirty,
-                               start_time);
+        __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
+                               bdi_thresh, bdi_dirty, start_time);
         spin_unlock(&bdi->wb.list_lock);
 }
 
 /*
+ * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
+ * will look to see if it needs to start dirty throttling.
+ *
+ * If dirty_poll_interval is too low, big NUMA machines will call the expensive
+ * global_page_state() too often. So scale it near-sqrt to the safety margin
+ * (the number of pages we may dirty without exceeding the dirty limits).
+ */
+static unsigned long dirty_poll_interval(unsigned long dirty,
+                                         unsigned long thresh)
+{
+        if (thresh > dirty)
+                return 1UL << (ilog2(thresh - dirty) >> 1);
+
+        return 1;
+}
+
+static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
+                                   unsigned long bdi_dirty)
+{
+        unsigned long bw = bdi->avg_write_bandwidth;
+        unsigned long hi = ilog2(bw);
+        unsigned long lo = ilog2(bdi->dirty_ratelimit);
+        unsigned long t;
+
+        /* target for 20ms max pause on 1-dd case */
+        t = HZ / 50;
+
+        /*
+         * Scale up pause time for concurrent dirtiers in order to reduce CPU
+         * overheads.
+         *
+         * (N * 20ms) on 2^N concurrent tasks.
+         */
+        if (hi > lo)
+                t += (hi - lo) * (20 * HZ) / 1024;
+
+        /*
+         * Limit pause time for small memory systems. If sleeping for too long
+         * time, a small pool of dirty/writeback pages may go empty and disk go
+         * idle.
+         *
+         * 8 serves as the safety ratio.
+         */
+        if (bdi_dirty)
+                t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+
+        /*
+         * The pause time will be settled within range (max_pause/4, max_pause).
+         * Apply a minimal value of 4 to get a non-zero max_pause/4.
+         */
+        return clamp_val(t, 4, MAX_PAUSE);
+}
+
+/*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
- * the caller to perform writeback if the system is over `vm_dirty_ratio'.
+ * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
  */
 static void balance_dirty_pages(struct address_space *mapping,
-                                unsigned long write_chunk)
+                                unsigned long pages_dirtied)
 {
-        unsigned long nr_reclaimable, bdi_nr_reclaimable;
+        unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
+        unsigned long bdi_reclaimable;
         unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
         unsigned long bdi_dirty;
+        unsigned long freerun;
         unsigned long background_thresh;
         unsigned long dirty_thresh;
         unsigned long bdi_thresh;
-        unsigned long task_bdi_thresh;
-        unsigned long min_task_bdi_thresh;
-        unsigned long pages_written = 0;
-        unsigned long pause = 1;
+        long pause = 0;
+        long uninitialized_var(max_pause);
         bool dirty_exceeded = false;
-        bool clear_dirty_exceeded = true;
+        unsigned long task_ratelimit;
+        unsigned long uninitialized_var(dirty_ratelimit);
+        unsigned long pos_ratio;
         struct backing_dev_info *bdi = mapping->backing_dev_info;
         unsigned long start_time = jiffies;
 
         for (;;) {
+                /*
+                 * Unstable writes are a feature of certain networked
+                 * filesystems (i.e. NFS) in which data may have been
+                 * written to the server's write cache, but has not yet
+                 * been flushed to permanent storage.
+                 */
                 nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                                         global_page_state(NR_UNSTABLE_NFS);
                 nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
@@ -681,12 +1039,28 @@ static void balance_dirty_pages(struct address_space *mapping,
                  * catch-up. This avoids (excessively) small writeouts
                  * when the bdi limits are ramping up.
                  */
-                if (nr_dirty <= (background_thresh + dirty_thresh) / 2)
+                freerun = dirty_freerun_ceiling(dirty_thresh,
+                                                background_thresh);
+                if (nr_dirty <= freerun)
                         break;
 
+                if (unlikely(!writeback_in_progress(bdi)))
+                        bdi_start_background_writeback(bdi);
+
+                /*
+                 * bdi_thresh is not treated as some limiting factor as
+                 * dirty_thresh, due to reasons
+                 * - in JBOD setup, bdi_thresh can fluctuate a lot
+                 * - in a system with HDD and USB key, the USB key may somehow
+                 *   go into state (bdi_dirty >> bdi_thresh) either because
+                 *   bdi_dirty starts high, or because bdi_thresh drops low.
+                 *   In this case we don't want to hard throttle the USB key
+                 *   dirtiers for 100 seconds until bdi_dirty drops under
+                 *   bdi_thresh. Instead the auxiliary bdi control line in
+                 *   bdi_position_ratio() will let the dirtier task progress
+                 *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
+                 */
                 bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
-                min_task_bdi_thresh = task_min_dirty_limit(bdi_thresh);
-                task_bdi_thresh = task_dirty_limit(current, bdi_thresh);
 
                 /*
                  * In order to avoid the stacked BDI deadlock we need
@@ -698,56 +1072,69 @@ static void balance_dirty_pages(struct address_space *mapping,
                  * actually dirty; with m+n sitting in the percpu
                  * deltas.
                  */
-                if (task_bdi_thresh < 2 * bdi_stat_error(bdi)) {
-                        bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
-                        bdi_dirty = bdi_nr_reclaimable +
+                if (bdi_thresh < 2 * bdi_stat_error(bdi)) {
+                        bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
+                        bdi_dirty = bdi_reclaimable +
                                     bdi_stat_sum(bdi, BDI_WRITEBACK);
                 } else {
-                        bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
-                        bdi_dirty = bdi_nr_reclaimable +
+                        bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
+                        bdi_dirty = bdi_reclaimable +
                                     bdi_stat(bdi, BDI_WRITEBACK);
                 }
 
-                /*
-                 * The bdi thresh is somehow "soft" limit derived from the
-                 * global "hard" limit. The former helps to prevent heavy IO
-                 * bdi or process from holding back light ones; The latter is
-                 * the last resort safeguard.
-                 */
-                dirty_exceeded = (bdi_dirty > task_bdi_thresh) ||
+                dirty_exceeded = (bdi_dirty > bdi_thresh) ||
                                   (nr_dirty > dirty_thresh);
-                clear_dirty_exceeded = (bdi_dirty <= min_task_bdi_thresh) &&
-                                        (nr_dirty <= dirty_thresh);
-
-                if (!dirty_exceeded)
-                        break;
-
-                if (!bdi->dirty_exceeded)
+                if (dirty_exceeded && !bdi->dirty_exceeded)
                         bdi->dirty_exceeded = 1;
 
-                bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty,
-                                     bdi_thresh, bdi_dirty, start_time);
+                bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
+                                     nr_dirty, bdi_thresh, bdi_dirty,
+                                     start_time);
 
-                /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
-                 * Unstable writes are a feature of certain networked
-                 * filesystems (i.e. NFS) in which data may have been
-                 * written to the server's write cache, but has not yet
-                 * been flushed to permanent storage.
-                 * Only move pages to writeback if this bdi is over its
-                 * threshold otherwise wait until the disk writes catch
-                 * up.
-                 */
-                trace_balance_dirty_start(bdi);
-                if (bdi_nr_reclaimable > task_bdi_thresh) {
-                        pages_written += writeback_inodes_wb(&bdi->wb,
-                                                             write_chunk);
-                        trace_balance_dirty_written(bdi, pages_written);
-                        if (pages_written >= write_chunk)
-                                break;          /* We've done our duty */
+                max_pause = bdi_max_pause(bdi, bdi_dirty);
+
+                dirty_ratelimit = bdi->dirty_ratelimit;
+                pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
+                                               background_thresh, nr_dirty,
+                                               bdi_thresh, bdi_dirty);
+                if (unlikely(pos_ratio == 0)) {
+                        pause = max_pause;
+                        goto pause;
+                }
+                task_ratelimit = (u64)dirty_ratelimit *
+                                        pos_ratio >> RATELIMIT_CALC_SHIFT;
+                pause = (HZ * pages_dirtied) / (task_ratelimit | 1);
+                if (unlikely(pause <= 0)) {
+                        trace_balance_dirty_pages(bdi,
+                                                  dirty_thresh,
+                                                  background_thresh,
+                                                  nr_dirty,
+                                                  bdi_thresh,
+                                                  bdi_dirty,
+                                                  dirty_ratelimit,
+                                                  task_ratelimit,
+                                                  pages_dirtied,
+                                                  pause,
+                                                  start_time);
+                        pause = 1; /* avoid resetting nr_dirtied_pause below */
+                        break;
                 }
+                pause = min(pause, max_pause);
+
+pause:
+                trace_balance_dirty_pages(bdi,
+                                          dirty_thresh,
+                                          background_thresh,
+                                          nr_dirty,
+                                          bdi_thresh,
+                                          bdi_dirty,
+                                          dirty_ratelimit,
+                                          task_ratelimit,
+                                          pages_dirtied,
+                                          pause,
+                                          start_time);
                 __set_current_state(TASK_UNINTERRUPTIBLE);
                 io_schedule_timeout(pause);
-                trace_balance_dirty_wait(bdi);
 
                 dirty_thresh = hard_dirty_limit(dirty_thresh);
                 /*
@@ -756,24 +1143,30 @@ static void balance_dirty_pages(struct address_space *mapping,
                  * 200ms is typically more than enough to curb heavy dirtiers;
                  * (b) the pause time limit makes the dirtiers more responsive.
                  */
-                if (nr_dirty < dirty_thresh &&
-                    bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2 &&
-                    time_after(jiffies, start_time + MAX_PAUSE))
+                if (nr_dirty < dirty_thresh)
                         break;
-
-                /*
-                 * Increase the delay for each loop, up to our previous
-                 * default of taking a 100ms nap.
-                 */
-                pause <<= 1;
-                if (pause > HZ / 10)
-                        pause = HZ / 10;
         }
 
-        /* Clear dirty_exceeded flag only when no task can exceed the limit */
-        if (clear_dirty_exceeded && bdi->dirty_exceeded)
+        if (!dirty_exceeded && bdi->dirty_exceeded)
                 bdi->dirty_exceeded = 0;
 
+        current->nr_dirtied = 0;
+        if (pause == 0) { /* in freerun area */
+                current->nr_dirtied_pause =
+                                dirty_poll_interval(nr_dirty, dirty_thresh);
+        } else if (pause <= max_pause / 4 &&
+                   pages_dirtied >= current->nr_dirtied_pause) {
+                current->nr_dirtied_pause = clamp_val(
+                                        dirty_ratelimit * (max_pause / 2) / HZ,
+                                        pages_dirtied + pages_dirtied / 8,
+                                        pages_dirtied * 4);
+        } else if (pause >= max_pause) {
+                current->nr_dirtied_pause = 1 | clamp_val(
+                                        dirty_ratelimit * (max_pause / 2) / HZ,
+                                        pages_dirtied / 4,
+                                        pages_dirtied - pages_dirtied / 8);
+        }
+
         if (writeback_in_progress(bdi))
                 return;
 
@@ -785,8 +1178,10 @@ static void balance_dirty_pages(struct address_space *mapping,
          * In normal mode, we start background writeout at the lower
          * background_thresh, to keep the amount of dirty memory low.
          */
-        if ((laptop_mode && pages_written) ||
-            (!laptop_mode && (nr_reclaimable > background_thresh)))
+        if (laptop_mode)
+                return;
+
+        if (nr_reclaimable > background_thresh)
                 bdi_start_background_writeback(bdi);
 }
 
@@ -800,7 +1195,7 @@ void set_page_dirty_balance(struct page *page, int page_mkwrite)
         }
 }
 
-static DEFINE_PER_CPU(unsigned long, bdp_ratelimits) = 0;
+static DEFINE_PER_CPU(int, bdp_ratelimits);
 
 /**
  * balance_dirty_pages_ratelimited_nr - balance dirty memory state
@@ -820,31 +1215,39 @@ void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
                                         unsigned long nr_pages_dirtied)
 {
         struct backing_dev_info *bdi = mapping->backing_dev_info;
-        unsigned long ratelimit;
-        unsigned long *p;
+        int ratelimit;
+        int *p;
 
         if (!bdi_cap_account_dirty(bdi))
                 return;
 
-        ratelimit = ratelimit_pages;
-        if (mapping->backing_dev_info->dirty_exceeded)
-                ratelimit = 8;
+        ratelimit = current->nr_dirtied_pause;
+        if (bdi->dirty_exceeded)
+                ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
+
+        current->nr_dirtied += nr_pages_dirtied;
 
+        preempt_disable();
         /*
-         * Check the rate limiting. Also, we do not want to throttle real-time
-         * tasks in balance_dirty_pages(). Period.
+         * This prevents one CPU to accumulate too many dirtied pages without
+         * calling into balance_dirty_pages(), which can happen when there are
+         * 1000+ tasks, all of them start dirtying pages at exactly the same
+         * time, hence all honoured too large initial task->nr_dirtied_pause.
          */
-        preempt_disable();
         p =  &__get_cpu_var(bdp_ratelimits);
-        *p += nr_pages_dirtied;
-        if (unlikely(*p >= ratelimit)) {
-                ratelimit = sync_writeback_pages(*p);
+        if (unlikely(current->nr_dirtied >= ratelimit))
                 *p = 0;
-                preempt_enable();
-                balance_dirty_pages(mapping, ratelimit);
-                return;
+        else {
+                *p += nr_pages_dirtied;
+                if (unlikely(*p >= ratelimit_pages)) {
+                        *p = 0;
+                        ratelimit = 0;
+                }
         }
         preempt_enable();
+
+        if (unlikely(current->nr_dirtied >= ratelimit))
+                balance_dirty_pages(mapping, current->nr_dirtied);
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
 
@@ -900,7 +1303,8 @@ void laptop_mode_timer_fn(unsigned long data)
          * threshold
          */
         if (bdi_has_dirty_io(&q->backing_dev_info))
-                bdi_start_writeback(&q->backing_dev_info, nr_pages);
+                bdi_start_writeback(&q->backing_dev_info, nr_pages,
+                                        WB_REASON_LAPTOP_TIMER);
 }
 
 /*
@@ -939,22 +1343,17 @@ void laptop_sync_completion(void)
  *
  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
- * thresholds before writeback cuts in.
- *
- * But the limit should not be set too high.  Because it also controls the
- * amount of memory which the balance_dirty_pages() caller has to write back.
- * If this is too large then the caller will block on the IO queue all the
- * time.  So limit it to four megabytes - the balance_dirty_pages() caller
- * will write six megabyte chunks, max.
+ * thresholds.
  */
 
 void writeback_set_ratelimit(void)
 {
-        ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);
+        unsigned long background_thresh;
+        unsigned long dirty_thresh;
+        global_dirty_limits(&background_thresh, &dirty_thresh);
+        ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
         if (ratelimit_pages < 16)
                 ratelimit_pages = 16;
-        if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
-                ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
 }
 
 static int __cpuinit
@@ -1324,6 +1723,7 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
                 __inc_zone_page_state(page, NR_FILE_DIRTY);
                 __inc_zone_page_state(page, NR_DIRTIED);
                 __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
+                __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
                 task_dirty_inc(current);
                 task_io_account_write(PAGE_CACHE_SIZE);
         }
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 132d1ddb2238..a1893c050795 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2266,7 +2266,8 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
                  */
                 writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
                 if (total_scanned > writeback_threshold) {
-                        wakeup_flusher_threads(laptop_mode ? 0 : total_scanned);
+                        wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
+                                                WB_REASON_TRY_TO_FREE_PAGES);
                         sc->may_writepage = 1;
                 }