On Fri, 23 Mar 2018 18:20:29 +0300 Andrey Ryabinin <aryabi...@virtuozzo.com> wrote:
> memcg reclaim may alter pgdat->flags based on the state of LRU lists > in cgroup and its children. PGDAT_WRITEBACK may force kswapd to sleep > congested_wait(), PGDAT_DIRTY may force kswapd to writeback filesystem > pages. But the worst here is PGDAT_CONGESTED, since it may force all > direct reclaims to stall in wait_iff_congested(). Note that only kswapd > have powers to clear any of these bits. This might just never happen if > cgroup limits configured that way. So all direct reclaims will stall > as long as we have some congested bdi in the system. > > Leave all pgdat->flags manipulations to kswapd. kswapd scans the whole > pgdat, only kswapd can clear pgdat->flags once node is balance, thus > it's reasonable to leave all decisions about node state to kswapd. > > Moving pgdat->flags manipulation to kswapd, means that cgroup2 recalim > now loses its congestion throttling mechanism. Add per-cgroup congestion > state and throttle cgroup2 reclaimers if memcg is in congestion state. > > Currently there is no need in per-cgroup PGDAT_WRITEBACK and PGDAT_DIRTY > bits since they alter only kswapd behavior. > > The problem could be easily demonstrated by creating heavy congestion > in one cgroup: > > echo "+memory" > /sys/fs/cgroup/cgroup.subtree_control > mkdir -p /sys/fs/cgroup/congester > echo 512M > /sys/fs/cgroup/congester/memory.max > echo $$ > /sys/fs/cgroup/congester/cgroup.procs > /* generate a lot of diry data on slow HDD */ > while true; do dd if=/dev/zero of=/mnt/sdb/zeroes bs=1M count=1024; done & > .... > while true; do dd if=/dev/zero of=/mnt/sdb/zeroes bs=1M count=1024; done & > > and some job in another cgroup: > > mkdir /sys/fs/cgroup/victim > echo 128M > /sys/fs/cgroup/victim/memory.max > > # time cat /dev/sda > /dev/null > real 10m15.054s > user 0m0.487s > sys 1m8.505s > > According to the tracepoint in wait_iff_congested(), the 'cat' spent 50% > of the time sleeping there. > > With the patch, cat don't waste time anymore: > > # time cat /dev/sda > /dev/null > real 5m32.911s > user 0m0.411s > sys 0m56.664s > Reviewers, please?
