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Date:   Mon, 27 Nov 2023 10:33:49 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Dmitry Rokosov <ddrokosov@salutedevices.com>
Cc:     rostedt@goodmis.org, mhiramat@kernel.org, hannes@cmpxchg.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, akpm@linux-foundation.org,
        kernel@sberdevices.ru, rockosov@gmail.com, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org
Subject: Re: [PATCH v3 2/2] mm: memcg: introduce new event to trace
 shrink_memcg
Message-ID: <ZWRifQgRR0570oDY@tiehlicka>
References: <20231123193937.11628-1-ddrokosov@salutedevices.com>
 <20231123193937.11628-3-ddrokosov@salutedevices.com>
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On Thu 23-11-23 22:39:37, Dmitry Rokosov wrote:
> The shrink_memcg flow plays a crucial role in memcg reclamation.
> Currently, it is not possible to trace this point from non-direct
> reclaim paths. However, direct reclaim has its own tracepoint, so there
> is no issue there. In certain cases, when debugging memcg pressure,
> developers may need to identify all potential requests for memcg
> reclamation including kswapd(). The patchset introduces the tracepoints
> mm_vmscan_memcg_shrink_{begin|end}() to address this problem.
> 
> Example of output in the kswapd context (non-direct reclaim):
>     kswapd0-39      [001] .....   240.356378: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.356396: mm_vmscan_memcg_shrink_end: nr_reclaimed=0 memcg=16
>     kswapd0-39      [001] .....   240.356420: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.356454: mm_vmscan_memcg_shrink_end: nr_reclaimed=1 memcg=16
>     kswapd0-39      [001] .....   240.356479: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.356506: mm_vmscan_memcg_shrink_end: nr_reclaimed=4 memcg=16
>     kswapd0-39      [001] .....   240.356525: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.356593: mm_vmscan_memcg_shrink_end: nr_reclaimed=11 memcg=16
>     kswapd0-39      [001] .....   240.356614: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.356738: mm_vmscan_memcg_shrink_end: nr_reclaimed=25 memcg=16
>     kswapd0-39      [001] .....   240.356790: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
>     kswapd0-39      [001] .....   240.357125: mm_vmscan_memcg_shrink_end: nr_reclaimed=53 memcg=16

In the previous version I have asked why do we need this specific
tracepoint when we already do have trace_mm_vmscan_lru_shrink_{in}active
which already give you a very good insight. That includes the number of
reclaimed pages but also more. I do see that we do not include memcg id
of the reclaimed LRU, but that shouldn't be a big problem to add, no?

-- 
Michal Hocko
SUSE Labs