linux-kernel - Re: [PATCH] mm, oom: allow oom reaper to race with exit

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Date:   Tue, 25 Jul 2017 16:26:17 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Oleg Nesterov <oleg@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

On Tue 25-07-17 17:17:23, Kirill A. Shutemov wrote:
[...]
> Below are numbers for the same test case, but from bigger machine (48
> threads, 64GiB of RAM).
> 
> v4.13-rc2:
> 
>  Performance counter stats for './a.sh 100000' (5 runs):
> 
>      159857.233790      task-clock:u (msec)       #    1.000 CPUs utilized            ( +-  3.21% )
>                  0      context-switches:u        #    0.000 K/sec
>                  0      cpu-migrations:u          #    0.000 K/sec
>          8,761,843      page-faults:u             #    0.055 M/sec                    ( +-  0.64% )
>     38,725,763,026      cycles:u                  #    0.242 GHz                      ( +-  0.18% )
>    272,691,643,016      stalled-cycles-frontend:u #  704.16% frontend cycles idle     ( +-  3.16% )
>     22,221,416,575      instructions:u            #    0.57  insn per cycle
>                                                   #   12.27  stalled cycles per insn  ( +-  0.00% )
>      5,306,829,649      branches:u                #   33.197 M/sec                    ( +-  0.00% )
>        240,783,599      branch-misses:u           #    4.54% of all branches          ( +-  0.15% )
> 
>      159.808721098 seconds time elapsed                                          ( +-  3.15% )
> 
> v4.13-rc2 + the patch:
> 
>  Performance counter stats for './a.sh 100000' (5 runs):
> 
>      167628.094556      task-clock:u (msec)       #    1.007 CPUs utilized            ( +-  1.63% )
>                  0      context-switches:u        #    0.000 K/sec
>                  0      cpu-migrations:u          #    0.000 K/sec
>          8,838,314      page-faults:u             #    0.053 M/sec                    ( +-  0.26% )
>     38,862,240,137      cycles:u                  #    0.232 GHz                      ( +-  0.10% )
>    282,105,057,553      stalled-cycles-frontend:u #  725.91% frontend cycles idle     ( +-  1.64% )
>     22,219,273,623      instructions:u            #    0.57  insn per cycle
>                                                   #   12.70  stalled cycles per insn  ( +-  0.00% )
>      5,306,165,194      branches:u                #   31.654 M/sec                    ( +-  0.00% )
>        240,473,075      branch-misses:u           #    4.53% of all branches          ( +-  0.07% )
> 
>      166.497005412 seconds time elapsed                                          ( +-  1.61% )
> 
> IMO, there is something to think about. ~4% slowdown is not insignificant.
> I expect effect to be bigger for larger machines.

Thanks for retesting Kirill. Are those numbers stable over runs? E.g.
the run without the patch has ~3% variance while the one with the patch
has it smaller. This sounds suspicious to me. There shouldn't be any
lock contention (except for the oom killer) so the lock shouldn't make
any difference wrt. variability.

Also I was about to post a more targeted test. Could you try it with it
as well, please?

-- 
Michal Hocko
SUSE Labs