writeback: switch to per-bdi threads for flushing data

This gets rid of pdflush for bdi writeout and kupdated style cleaning.
pdflush writeout suffers from lack of locality and also requires more
threads to handle the same workload, since it has to work in a
non-blocking fashion against each queue. This also introduces lumpy
behaviour and potential request starvation, since pdflush can be starved
for queue access if others are accessing it. A sample ffsb workload that
does random writes to files is about 8% faster here on a simple SATA drive
during the benchmark phase. File layout also seems a LOT more smooth in
vmstat:

 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 0  1      0 608848   2652 375372    0    0     0 71024  604    24  1 10 48 42
 0  1      0 549644   2712 433736    0    0     0 60692  505    27  1  8 48 44
 1  0      0 476928   2784 505192    0    0     4 29540  553    24  0  9 53 37
 0  1      0 457972   2808 524008    0    0     0 54876  331    16  0  4 38 58
 0  1      0 366128   2928 614284    0    0     4 92168  710    58  0 13 53 34
 0  1      0 295092   3000 684140    0    0     0 62924  572    23  0  9 53 37
 0  1      0 236592   3064 741704    0    0     4 58256  523    17  0  8 48 44
 0  1      0 165608   3132 811464    0    0     0 57460  560    21  0  8 54 38
 0  1      0 102952   3200 873164    0    0     4 74748  540    29  1 10 48 41
 0  1      0  48604   3252 926472    0    0     0 53248  469    29  0  7 47 45

where vanilla tends to fluctuate a lot in the creation phase:

 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 1  1      0 678716   5792 303380    0    0     0 74064  565    50  1 11 52 36
 1  0      0 662488   5864 319396    0    0     4   352  302   329  0  2 47 51
 0  1      0 599312   5924 381468    0    0     0 78164  516    55  0  9 51 40
 0  1      0 519952   6008 459516    0    0     4 78156  622    56  1 11 52 37
 1  1      0 436640   6092 541632    0    0     0 82244  622    54  0 11 48 41
 0  1      0 436640   6092 541660    0    0     0     8  152    39  0  0 51 49
 0  1      0 332224   6200 644252    0    0     4 102800  728    46  1 13 49 36
 1  0      0 274492   6260 701056    0    0     4 12328  459    49  0  7 50 43
 0  1      0 211220   6324 763356    0    0     0 106940  515    37  1 10 51 39
 1  0      0 160412   6376 813468    0    0     0  8224  415    43  0  6 49 45
 1  1      0  85980   6452 886556    0    0     4 113516  575    39  1 11 54 34
 0  2      0  85968   6452 886620    0    0     0  1640  158   211  0  0 46 54

A 10 disk test with btrfs performs 26% faster with per-bdi flushing. A
SSD based writeback test on XFS performs over 20% better as well, with
the throughput being very stable around 1GB/sec, where pdflush only
manages 750MB/sec and fluctuates wildly while doing so. Random buffered
writes to many files behave a lot better as well, as does random mmap'ed
writes.

A separate thread is added to sync the super blocks. In the long term,
adding sync_supers_bdi() functionality could get rid of this thread again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 28 insertions, 151 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f8341b6019b..25e7770309b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -36,15 +36,6 @@
 #include <linux/pagevec.h>
 
 /*
- * The maximum number of pages to writeout in a single bdflush/kupdate
- * operation.  We do this so we don't hold I_SYNC against an inode for
- * enormous amounts of time, which would block a userspace task which has
- * been forced to throttle against that inode.  Also, the code reevaluates
- * the dirty each time it has written this many pages.
- */
-#define MAX_WRITEBACK_PAGES     1024
-
-/*
  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
  * will look to see if it needs to force writeback or throttling.
  */
@@ -117,8 +108,6 @@ EXPORT_SYMBOL(laptop_mode);
 /* End of sysctl-exported parameters */
 
 
-static void background_writeout(unsigned long _min_pages);
-
 /*
  * Scale the writeback cache size proportional to the relative writeout speeds.
  *
@@ -326,7 +315,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
         int ret = 0;
 
-        mutex_lock(&bdi_lock);
+        spin_lock(&bdi_lock);
         if (min_ratio > bdi->max_ratio) {
                 ret = -EINVAL;
         } else {
@@ -338,7 +327,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
                         ret = -EINVAL;
                 }
         }
-        mutex_unlock(&bdi_lock);
+        spin_unlock(&bdi_lock);
 
         return ret;
 }
@@ -350,14 +339,14 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
         if (max_ratio > 100)
                 return -EINVAL;
 
-        mutex_lock(&bdi_lock);
+        spin_lock(&bdi_lock);
         if (bdi->min_ratio > max_ratio) {
                 ret = -EINVAL;
         } else {
                 bdi->max_ratio = max_ratio;
                 bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100;
         }
-        mutex_unlock(&bdi_lock);
+        spin_unlock(&bdi_lock);
 
         return ret;
 }
@@ -543,7 +532,7 @@ static void balance_dirty_pages(struct address_space *mapping)
                  * up.
                  */
                 if (bdi_nr_reclaimable > bdi_thresh) {
-                        writeback_inodes(&wbc);
+                        writeback_inodes_wbc(&wbc);
                         pages_written += write_chunk - wbc.nr_to_write;
                         get_dirty_limits(&background_thresh, &dirty_thresh,
                                        &bdi_thresh, bdi);
@@ -572,7 +561,7 @@ static void balance_dirty_pages(struct address_space *mapping)
                 if (pages_written >= write_chunk)
                         break;          /* We've done our duty */
 
-                congestion_wait(BLK_RW_ASYNC, HZ/10);
+                schedule_timeout(1);
         }
 
         if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
@@ -591,10 +580,18 @@ static void balance_dirty_pages(struct address_space *mapping)
          * background_thresh, to keep the amount of dirty memory low.
          */
         if ((laptop_mode && pages_written) ||
-                        (!laptop_mode && (global_page_state(NR_FILE_DIRTY)
-                                          + global_page_state(NR_UNSTABLE_NFS)
-                                          > background_thresh)))
-                pdflush_operation(background_writeout, 0);
+            (!laptop_mode && ((nr_writeback = global_page_state(NR_FILE_DIRTY)
+                                          + global_page_state(NR_UNSTABLE_NFS))
+                                          > background_thresh))) {
+                struct writeback_control wbc = {
+                        .bdi            = bdi,
+                        .sync_mode      = WB_SYNC_NONE,
+                        .nr_to_write    = nr_writeback,
+                };
+
+
+                bdi_start_writeback(&wbc);
+        }
 }
 
 void set_page_dirty_balance(struct page *page, int page_mkwrite)
@@ -678,153 +675,35 @@ void throttle_vm_writeout(gfp_t gfp_mask)
         }
 }
 
-/*
- * writeback at least _min_pages, and keep writing until the amount of dirty
- * memory is less than the background threshold, or until we're all clean.
- */
-static void background_writeout(unsigned long _min_pages)
-{
-        long min_pages = _min_pages;
-        struct writeback_control wbc = {
-                .bdi            = NULL,
-                .sync_mode      = WB_SYNC_NONE,
-                .older_than_this = NULL,
-                .nr_to_write    = 0,
-                .nonblocking    = 1,
-                .range_cyclic   = 1,
-        };
-
-        for ( ; ; ) {
-                unsigned long background_thresh;
-                unsigned long dirty_thresh;
-
-                get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
-                if (global_page_state(NR_FILE_DIRTY) +
-                        global_page_state(NR_UNSTABLE_NFS) < background_thresh
-                                && min_pages <= 0)
-                        break;
-                wbc.more_io = 0;
-                wbc.encountered_congestion = 0;
-                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
-                wbc.pages_skipped = 0;
-                writeback_inodes(&wbc);
-                min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
-                if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
-                        /* Wrote less than expected */
-                        if (wbc.encountered_congestion || wbc.more_io)
-                                congestion_wait(BLK_RW_ASYNC, HZ/10);
-                        else
-                                break;
-                }
-        }
-}
-
-/*
- * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
- * the whole world.  Returns 0 if a pdflush thread was dispatched.  Returns
- * -1 if all pdflush threads were busy.
- */
-int wakeup_pdflush(long nr_pages)
-{
-        if (nr_pages == 0)
-                nr_pages = global_page_state(NR_FILE_DIRTY) +
-                                global_page_state(NR_UNSTABLE_NFS);
-        return pdflush_operation(background_writeout, nr_pages);
-}
-
-static void wb_timer_fn(unsigned long unused);
 static void laptop_timer_fn(unsigned long unused);
 
-static DEFINE_TIMER(wb_timer, wb_timer_fn, 0, 0);
 static DEFINE_TIMER(laptop_mode_wb_timer, laptop_timer_fn, 0, 0);
 
 /*
- * Periodic writeback of "old" data.
- *
- * Define "old": the first time one of an inode's pages is dirtied, we mark the
- * dirtying-time in the inode's address_space.  So this periodic writeback code
- * just walks the superblock inode list, writing back any inodes which are
- * older than a specific point in time.
- *
- * Try to run once per dirty_writeback_interval.  But if a writeback event
- * takes longer than a dirty_writeback_interval interval, then leave a
- * one-second gap.
- *
- * older_than_this takes precedence over nr_to_write.  So we'll only write back
- * all dirty pages if they are all attached to "old" mappings.
- */
-static void wb_kupdate(unsigned long arg)
-{
-        unsigned long oldest_jif;
-        unsigned long start_jif;
-        unsigned long next_jif;
-        long nr_to_write;
-        struct writeback_control wbc = {
-                .bdi            = NULL,
-                .sync_mode      = WB_SYNC_NONE,
-                .older_than_this = &oldest_jif,
-                .nr_to_write    = 0,
-                .nonblocking    = 1,
-                .for_kupdate    = 1,
-                .range_cyclic   = 1,
-        };
-
-        sync_supers();
-
-        oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval * 10);
-        start_jif = jiffies;
-        next_jif = start_jif + msecs_to_jiffies(dirty_writeback_interval * 10);
-        nr_to_write = global_page_state(NR_FILE_DIRTY) +
-                        global_page_state(NR_UNSTABLE_NFS) +
-                        (inodes_stat.nr_inodes - inodes_stat.nr_unused);
-        while (nr_to_write > 0) {
-                wbc.more_io = 0;
-                wbc.encountered_congestion = 0;
-                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
-                writeback_inodes(&wbc);
-                if (wbc.nr_to_write > 0) {
-                        if (wbc.encountered_congestion || wbc.more_io)
-                                congestion_wait(BLK_RW_ASYNC, HZ/10);
-                        else
-                                break;  /* All the old data is written */
-                }
-                nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
-        }
-        if (time_before(next_jif, jiffies + HZ))
-                next_jif = jiffies + HZ;
-        if (dirty_writeback_interval)
-                mod_timer(&wb_timer, next_jif);
-}
-
-/*
  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
  */
 int dirty_writeback_centisecs_handler(ctl_table *table, int write,
         struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
 {
         proc_dointvec(table, write, file, buffer, length, ppos);
-        if (dirty_writeback_interval)
-                mod_timer(&wb_timer, jiffies +
-                        msecs_to_jiffies(dirty_writeback_interval * 10));
-        else
-                del_timer(&wb_timer);
         return 0;
 }
 
-static void wb_timer_fn(unsigned long unused)
-{
-        if (pdflush_operation(wb_kupdate, 0) < 0)
-                mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */
-}
-
-static void laptop_flush(unsigned long unused)
+static void do_laptop_sync(struct work_struct *work)
 {
-        sys_sync();
+        wakeup_flusher_threads(0);
+        kfree(work);
 }
 
 static void laptop_timer_fn(unsigned long unused)
 {
-        pdflush_operation(laptop_flush, 0);
+        struct work_struct *work;
+
+        work = kmalloc(sizeof(*work), GFP_ATOMIC);
+        if (work) {
+                INIT_WORK(work, do_laptop_sync);
+                schedule_work(work);
+        }
 }
 
 /*
@@ -907,8 +786,6 @@ void __init page_writeback_init(void)
 {
         int shift;
 
-        mod_timer(&wb_timer,
-                  jiffies + msecs_to_jiffies(dirty_writeback_interval * 10));
         writeback_set_ratelimit();
         register_cpu_notifier(&ratelimit_nb);