[PATCH v2 2/4] mm: correct calculation of wb's bg_thresh in cgroup domain

[Kemeng Shi]

The wb_calc_thresh is supposed to calculate wb's share of bg_thresh in
global domain. To calculate wb's share of bg_thresh in cgroup domain,
it's more reasonable to use __wb_calc_thresh in which way we calculate
dirty_thresh in cgroup domain in balance_dirty_pages().

Consider following domain hierarchy:
                global domain (> 20G)
                /                 \
        cgroup domain1(10G)     cgroup domain2(10G)
                |                 |
bdi            wb1               wb2
Assume wb1 and wb2 has the same bandwidth.
We have global domain bg_thresh > 2G, cgroup domain bg_thresh 1G.
Then we have:
wb's thresh in global domain = 2G * (wb bandwidth) / (system bandwidth)
= 2G * 1/2 = 1G
wb's thresh in cgroup domain = 1G * (wb bandwidth) / (system bandwidth)
= 1G * 1/2 = 0.5G
At last, wb1 and wb2 will be limited at 0.5G, the system will be limited
at 1G which is less than global domain bg_thresh 2G.

Test as following:
/* make it easier to observe the issue */
echo 300000 > /proc/sys/vm/dirty_expire_centisecs
echo 100 > /proc/sys/vm/dirty_writeback_centisecs

/* run fio in wb1 */
cd /sys/fs/cgroup
echo "+memory +io" > cgroup.subtree_control
mkdir group1
cd group1
echo 10G > memory.high
echo 10G > memory.max
echo $$ > cgroup.procs
mkfs.ext4 -F /dev/vdb
mount /dev/vdb /bdi1/
fio -name test -filename=/bdi1/file -size=600M -ioengine=libaio -bs=4K \
-iodepth=1 -rw=write -direct=0 --time_based -runtime=600 -invalidate=0

/* run fio in wb2 with a new shell */
cd /sys/fs/cgroup
mkdir group2
cd group2
echo 10G > memory.high
echo 10G > memory.max
echo $$ > cgroup.procs
mkfs.ext4 -F /dev/vdc
mount /dev/vdc /bdi2/
fio -name test -filename=/bdi2/file -size=600M -ioengine=libaio -bs=4K \
-iodepth=1 -rw=write -direct=0 --time_based -runtime=600 -invalidate=0

Before fix, the wrttien pages of wb1 and wb2 reported from
toos/writeback/wb_monitor.py keep growing. After fix, rare written pages
are accumulated.
There is no obvious change in fio result.

Fixes: 74d369443325 ("writeback: Fix performance regression in wb_over_bg_thresh()")
Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 mm/page-writeback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2a3b68aae336..14893b20d38c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2137,7 +2137,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 		if (mdtc->dirty > mdtc->bg_thresh)
 			return true;
 
-		thresh = wb_calc_thresh(mdtc->wb, mdtc->bg_thresh);
+		thresh = __wb_calc_thresh(mdtc, mdtc->bg_thresh);
 		if (thresh < 2 * wb_stat_error())
 			reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
 		else
-- 
2.30.0