linux-kernel - Re: [PATCH v2 0/5] Introduce memcg_stock

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Date:   Wed, 25 Jan 2023 09:33:49 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Leonardo Bras <leobras@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marcelo Tosatti <mtosatti@...hat.com>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining

On Wed 25-01-23 04:34:57, Leonardo Bras wrote:
> Disclaimer:
> a - The cover letter got bigger than expected, so I had to split it in
>     sections to better organize myself. I am not very confortable with it.
> b - Performance numbers below did not include patch 5/5 (Remove flags
>     from memcg_stock_pcp), which could further improve performance for
>     drain_all_stock(), but I could only notice the optimization at the
>     last minute.
> 
> 
> 0 - Motivation:
> On current codebase, when drain_all_stock() is ran, it will schedule a
> drain_local_stock() for each cpu that has a percpu stock associated with a
> descendant of a given root_memcg.
> 
> This happens even on 'isolated cpus', a feature commonly used on workloads that
> are sensitive to interruption and context switching such as vRAN and Industrial
> Control Systems.
> 
> Since this scheduling behavior is a problem to those workloads, the proposal is
> to replace the current local_lock + schedule_work_on() solution with a per-cpu
> spinlock.

If IIRC we have also discussed that isolated CPUs can simply opt out
from the pcp caching and therefore the problem would be avoided
altogether without changes to the locking scheme. I do not see anything
regarding that in this submission. Could you elaborate why you have
abandoned this option?
-- 
Michal Hocko
SUSE Labs