linux-kernel - Re: [PATCH 2/2] mm: memcg: add a new MEMCG_UPDATE

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Date:   Wed, 6 Jan 2021 12:45:50 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Chris Down <chris@...isdown.name>, Roman Gushchin <guro@...com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, andi.kleen@...el.com,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH 2/2] mm: memcg: add a new MEMCG_UPDATE_BATCH

Hi Chris,

On Wed, Jan 06, 2021 at 03:43:36AM +0000, Chris Down wrote:
> Feng Tang writes:
> >One further thought is, there are quite some "BATCH" number in
> >kernel for perf-cpu/global data updating, maybe we can add a
> >global flag 'sysctl_need_accurate_stats' for
> >	if (sysctl_need_accurate_stats)
> >		batch = SMALLER_BATCH
> >	else
> >		batch = BIGGER_BATCH
> 
> Moving decisions like this to the system administrator is not really a
> solution to the problem -- inclusion should at least be contingent on either
> having "correct-ish" stats exported to userspace. Displaying broken stats to
> the user -- even with a configuration knob -- is less than ideal and is
> likely to confuse and confound issues in future.
> 
> I would also like to see numbers from more real-world workloads.

Sure. Roman also mentioned this. Do you have some suggestions for the
workload or benchmarks? I don't have much knowledge on this, and have
only leveraged some of 0day's benchmarking systems.

Thanks,
Feng

> MEMCG_CHARGE_BATCH is certainly fairly arbitrary as-is, but if it is going
> to be changed, the reason for that change and its implications (positive and
> negative) for real-world workloads must be well understood, and I'm not sure
> we're there yet.