writeback: move global_dirty_limit into wb_domain

This patch is a part of the series to define wb_domain which
represents a domain that wb's (bdi_writeback's) belong to and are
measured against each other in.  This will enable IO backpressure
propagation for cgroup writeback.

global_dirty_limit exists to regulate the global dirty threshold which
is a property of the wb_domain.  This patch moves hard_dirty_limit,
dirty_lock, and update_time into wb_domain.

This is pure reorganization and doesn't introduce any behavioral
changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

4 files changed, 44 insertions, 28 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index cd89484486f6..51c8a5b14cdf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -887,7 +887,7 @@ static long writeback_chunk_size(struct bdi_writeback *wb,
                 pages = LONG_MAX;
         else {
                 pages = min(wb->avg_write_bandwidth / 2,
-                            global_dirty_limit / DIRTY_SCOPE);
+                            global_wb_domain.dirty_limit / DIRTY_SCOPE);
                 pages = min(pages, work->nr_pages);
                 pages = round_down(pages + MIN_WRITEBACK_PAGES,
                                    MIN_WRITEBACK_PAGES);
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 3148db1296a2..5fdd4e1805e6 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -95,6 +95,8 @@ struct writeback_control {
  * dirtyable memory accordingly.
  */
 struct wb_domain {
+        spinlock_t lock;
+
         /*
          * Scale the writeback cache size proportional to the relative
          * writeout speed.
@@ -115,6 +117,19 @@ struct wb_domain {
         struct fprop_global completions;
         struct timer_list period_timer; /* timer for aging of completions */
         unsigned long period_time;
+
+        /*
+         * The dirtyable memory and dirty threshold could be suddenly
+         * knocked down by a large amount (eg. on the startup of KVM in a
+         * swapless system). This may throw the system into deep dirty
+         * exceeded state and throttle heavy/light dirtiers alike. To
+         * retain good responsiveness, maintain global_dirty_limit for
+         * tracking slowly down to the knocked down dirty threshold.
+         *
+         * Both fields are protected by ->lock.
+         */
+        unsigned long dirty_limit_tstamp;
+        unsigned long dirty_limit;
 };
 
 /*
@@ -153,7 +168,7 @@ void throttle_vm_writeout(gfp_t gfp_mask);
 bool zone_dirty_ok(struct zone *zone);
 int wb_domain_init(struct wb_domain *dom, gfp_t gfp);
 
-extern unsigned long global_dirty_limit;
+extern struct wb_domain global_wb_domain;
 
 /* These are exported to sysctl. */
 extern int dirty_background_ratio;
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 9b876f6cc81a..bec69995968f 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -361,7 +361,7 @@ TRACE_EVENT(global_dirty_state,
                 __entry->nr_written     = global_page_state(NR_WRITTEN);
                 __entry->background_thresh = background_thresh;
                 __entry->dirty_thresh   = dirty_thresh;
-                __entry->dirty_limit = global_dirty_limit;
+                __entry->dirty_limit    = global_wb_domain.dirty_limit;
         ),
 
         TP_printk("dirty=%lu writeback=%lu unstable=%lu "
@@ -463,8 +463,9 @@ TRACE_EVENT(balance_dirty_pages,
                 unsigned long freerun = (thresh + bg_thresh) / 2;
                 strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
 
-                __entry->limit          = global_dirty_limit;
-                __entry->setpoint       = (global_dirty_limit + freerun) / 2;
+                __entry->limit          = global_wb_domain.dirty_limit;
+                __entry->setpoint       = (global_wb_domain.dirty_limit +
+                                                freerun) / 2;
                 __entry->dirty          = dirty;
                 __entry->bdi_setpoint   = __entry->setpoint *
                                                 bdi_thresh / (thresh + 1);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 08e1737edb39..27e60ba8e688 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -122,9 +122,7 @@ EXPORT_SYMBOL(laptop_mode);
 
 /* End of sysctl-exported parameters */
 
-unsigned long global_dirty_limit;
-
-static struct wb_domain global_wb_domain;
+struct wb_domain global_wb_domain;
 
 /*
  * Length of period for aging writeout fractions of bdis. This is an
@@ -470,9 +468,15 @@ static void writeout_period(unsigned long t)
 int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
 {
         memset(dom, 0, sizeof(*dom));
+
+        spin_lock_init(&dom->lock);
+
         init_timer_deferrable(&dom->period_timer);
         dom->period_timer.function = writeout_period;
         dom->period_timer.data = (unsigned long)dom;
+
+        dom->dirty_limit_tstamp = jiffies;
+
         return fprop_global_init(&dom->completions, gfp);
 }
 
@@ -532,7 +536,9 @@ static unsigned long dirty_freerun_ceiling(unsigned long thresh,
 
 static unsigned long hard_dirty_limit(unsigned long thresh)
 {
-        return max(thresh, global_dirty_limit);
+        struct wb_domain *dom = &global_wb_domain;
+
+        return max(thresh, dom->dirty_limit);
 }
 
 /**
@@ -916,17 +922,10 @@ out:
         wb->avg_write_bandwidth = avg;
 }
 
-/*
- * The global dirtyable memory and dirty threshold could be suddenly knocked
- * down by a large amount (eg. on the startup of KVM in a swapless system).
- * This may throw the system into deep dirty exceeded state and throttle
- * heavy/light dirtiers alike. To retain good responsiveness, maintain
- * global_dirty_limit for tracking slowly down to the knocked down dirty
- * threshold.
- */
 static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
 {
-        unsigned long limit = global_dirty_limit;
+        struct wb_domain *dom = &global_wb_domain;
+        unsigned long limit = dom->dirty_limit;
 
         /*
          * Follow up in one step.
@@ -939,7 +938,7 @@ static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
         /*
          * Follow down slowly. Use the higher one as the target, because thresh
          * may drop below dirty. This is exactly the reason to introduce
-         * global_dirty_limit which is guaranteed to lie above the dirty pages.
+         * dom->dirty_limit which is guaranteed to lie above the dirty pages.
          */
         thresh = max(thresh, dirty);
         if (limit > thresh) {
@@ -948,28 +947,27 @@ static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
         }
         return;
 update:
-        global_dirty_limit = limit;
+        dom->dirty_limit = limit;
 }
 
 static void global_update_bandwidth(unsigned long thresh,
                                     unsigned long dirty,
                                     unsigned long now)
 {
-        static DEFINE_SPINLOCK(dirty_lock);
-        static unsigned long update_time = INITIAL_JIFFIES;
+        struct wb_domain *dom = &global_wb_domain;
 
         /*
          * check locklessly first to optimize away locking for the most time
          */
-        if (time_before(now, update_time + BANDWIDTH_INTERVAL))
+        if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
                 return;
 
-        spin_lock(&dirty_lock);
+        spin_lock(&dom->lock);