linux-kernel - Re: [PATCH v2 2/2] mm: memcg: introduce new event to trace shrink

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Message-ID: <ZV4BK0wbUAZBIhmA@tiehlicka>
Date:   Wed, 22 Nov 2023 14:24:59 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Dmitry Rokosov <ddrokosov@...utedevices.com>
Cc:     rostedt@...dmis.org, mhiramat@...nel.org, hannes@...xchg.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, akpm@...ux-foundation.org,
        kernel@...rdevices.ru, rockosov@...il.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: memcg: introduce new event to trace
 shrink_memcg

On Wed 22-11-23 13:58:36, Dmitry Rokosov wrote:
> Hello Michal,
> 
> Thank you for the quick review!
> 
> On Wed, Nov 22, 2023 at 11:23:24AM +0100, Michal Hocko wrote:
> > On Wed 22-11-23 13:01:56, 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.
> > 
> > Is this really true? AFAICS we have
> > mm_vmscan_lru_isolate
> > mm_vmscan_lru_shrink_active
> > mm_vmscan_lru_shrink_inactive
> > 
> > which are in the vry core of the memory reclaim. Sure post processing
> > those is some work.
> 
> Sure, you are absolutely right. In the usual scenario, the memcg
> shrinker utilizes two sub-shrinkers: slab and LRU. We can enable the
> tracepoints you mentioned and analyze them. However, there is one
> potential issue. Enabling these tracepoints will trigger the reclaim
> events show for all pages. Although we can filter them per pid, we
> cannot filter them per cgroup. Nevertheless, there are times when it
> would be extremely beneficial to comprehend the effectiveness of the
> reclaim process within the relevant cgroup. For this reason, I am adding
> the cgroup name to the memcg tracepoints and implementing a cumulative
> tracepoint for memcg shrink (LRU + slab)."

I can see how printing memcg in mm_vmscan_memcg_reclaim_begin makes it
easier to postprocess per memcg reclaim. But you could do that just by
adding that to mm_vmscan_memcg_reclaim_{begin, end}, no? Why exactly
does this matter for kswapd and other global reclaim contexts? 
-- 
Michal Hocko
SUSE Labs