Re: [PATCH 6/6] mm/vmscan: Don't mess with pgdat->flags in memcg reclaim.

Andrey Ryabinin
2018-03-21T11:14:35Z

On 03/20/2018 06:29 PM, Michal Hocko wrote:

>> Leave all pgdat->flags manipulations to kswapd. kswapd scans the whole
>> pgdat, so it's reasonable to leave all decisions about node stat
>> to kswapd. Also add per-cgroup congestion state to avoid needlessly
>> burning CPU in cgroup reclaim if heavy congestion is observed.
>>
>> 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
>>
>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>> ---
>> include/linux/backing-dev.h | 2 +-
>> include/linux/memcontrol.h | 2 ++
>> mm/backing-dev.c | 19 ++++------
>> mm/vmscan.c | 84 ++++++++++++++++++++++++++++++++-------------
>> 4 files changed, 70 insertions(+), 37 deletions(-)
>
> This patch seems overly complicated. Why don't you simply reduce the whole
> pgdat_flags handling to global_reclaim()?
>

In that case cgroup2 reclaim wouldn't have any way of throttling if cgroup is full of congested dirty pages.