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>

2 files changed, 20 insertions, 4 deletions

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);