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

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Date:   Wed, 21 Mar 2018 14:14:35 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH 6/6] mm/vmscan: Don't mess with pgdat->flags in memcg
 reclaim.



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@...tuozzo.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.