From fabd2e42bc71d78d1e9dcd5af453d1ab86cc2a7e Mon Sep 17 00:00:00 2001 From: Kemeng Shi Date: Thu, 25 Apr 2024 21:17:22 +0800 Subject: mm: correct calculation of wb's bg_thresh in cgroup domain wb_calc_thresh() is calculating wb's share of bg_thresh in the global domain. However in case of cgroup writeback this is not the right thing to do. Consider the following domain hierarchy: global domain (> 20G) / \ cgroup1 (10G) cgroup2 (10G) | | bdi wb1 wb2 and assume wb1 and wb2 have the same bandwidth and the background threshold is set at 10%. The bg_thresh of cgroup1 and cgroup2 is going to be 1G. Now because wb_calc_thresh(mdtc->wb, mdtc->bg_thresh) calculates per-wb threshold in the global domain as (wb bandwidth) / (domain bandwidth) it returns bg_thresh for wb1 as 0.5G although it has nobody to compete against in cgroup1. Fix the problem by calculating wb's share of bg_thresh in the cgroup domain. 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. [jack@suse.cz: changelog rewording] Link: https://lkml.kernel.org/r/20240425131724.36778-3-shikemeng@huaweicloud.com Fixes: 74d369443325 ("writeback: Fix performance regression in wb_over_bg_thresh()") Signed-off-by: Kemeng Shi Reviewed-by: Jan Kara Cc: Howard Cochran Cc: Jens Axboe Cc: Matthew Wilcox (Oracle) Cc: Miklos Szeredi Cc: Tejun Heo Signed-off-by: Andrew Morton --- mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'mm/page-writeback.c') 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 -- cgit v1.2.3