[PATCH 2/2] fs/writeback: do memory cgroup related writeback firstly

[Li RongQing]

When a machine has hundreds of memory cgroups, and some cgroups
generate more or less dirty pages, but a cgroup of them has lots
of memory pressure and always tries to reclaim dirty page, then it
will trigger all cgroups to writeback, which is less efficient:

1.if the used memory in a memory cgroup reaches its limit,
it is useless to writeback other cgroups.
2.other cgroups can wait more time to merge write request

so replace the full flush with flushing writeback of memory cgroup
whose tasks tries to reclaim memory and trigger writeback, if
nothing is writeback, then fallback a full flush

After this patch, the writing performance enhance 5% in below setup:
  $mount -t cgroup none -o memory /cgroups/memory/
  $mkdir /cgroups/memory/x1
  $echo $$ > /cgroups/memory/x1/tasks
  $echo 100M > /cgroups/memory/x1/memory.limit_in_bytes
  $cd /cgroups/memory/
  $seq 10000|xargs  mkdir
  $fio -filename=/home/test1 -direct=0 -iodepth 1 -thread -rw=write -ioengine=libaio -bs=16k -size=20G
Before:
WRITE: io=20480MB, aggrb=779031KB/s, minb=779031KB/s, maxb=779031KB/s, mint=26920msec, maxt=26920msec
After:
WRITE: io=20480MB, aggrb=831708KB/s, minb=831708KB/s, maxb=831708KB/s, mint=25215msec, maxt=25215msec

And this patch can reduce io util in this condition, like there
is two disks, one disks is used to store all kinds of logs, it
should be less io pressure, and other is used to store hadoop data
which will write lots of data to disk, but both disk io utils are
high in fact, since when hadoop reclaims memory, it will wake all
memory cgroup writeback.

Signed-off-by: Zhang Yu <zhangyu31@xxxxxxxxx>
Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
---
 fs/fs-writeback.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 471d863958bc..475cada5d1cf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -35,6 +35,11 @@
  */
 #define MIN_WRITEBACK_PAGES	(4096UL >> (PAGE_SHIFT - 10))
 
+/*
+ * if WB cgroup dirty pages is bigger than it, not start a full flush
+ */
+#define MIN_WB_DIRTY_PAGES 64
+
 struct wb_completion {
 	atomic_t		cnt;
 };
@@ -2005,6 +2010,32 @@ void wakeup_flusher_threads(enum wb_reason reason)
 	if (blk_needs_flush_plug(current))
 		blk_schedule_flush_plug(current);
 
+#ifdef CONFIG_CGROUP_WRITEBACK
+	if (reason == WB_REASON_VMSCAN) {
+		unsigned long tmp, pdirty = 0;
+
+		rcu_read_lock();
+		list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
+			struct bdi_writeback *wb = wb_find_current(bdi);
+
+			if (wb) {
+				tmp = mem_cgroup_wb_dirty_stats(wb);
+				if (tmp) {
+					pdirty += tmp;
+					wb_start_writeback(wb, reason);
+
+					if (wb == &bdi->wb)
+						pdirty += MIN_WB_DIRTY_PAGES;
+				}
+			}
+		}
+		rcu_read_unlock();
+
+		if (pdirty > MIN_WB_DIRTY_PAGES)
+			return;
+	}
+#endif
+
 	rcu_read_lock();
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
 		__wakeup_flusher_threads_bdi(bdi, reason);
-- 
2.16.2