netdev - Re: [PATCH 3/3] memcg: increase MEMCG_CHARGE

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Message-ID: <YwPZ1lpJ98pZSLmw@dhcp22.suse.cz>
Date:   Mon, 22 Aug 2022 21:34:59 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Koutný <mkoutny@...e.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Oliver Sang <oliver.sang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, lkp@...ts.01.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] memcg: increase MEMCG_CHARGE_BATCH to 64

On Mon 22-08-22 11:37:30, Roman Gushchin wrote:
[...]
> I wonder only if we want to make it configurable (Idk a sysctl or maybe
> a config option) and close the topic.

I do not think this is a good idea. We have other examples where we have
outsourced internal tunning to the userspace and it has mostly proven
impractical and long term more problematic than useful (e.g.
lowmem_reserve_ratio, percpu_pagelist_high_fraction, swappiness just to
name some that come to my mind). I have seen more often these to be used
incorrectly than useful.

In this case, I guess we should consider either moving to per memcg
charge batching and see whether the pcp overhead x memcg_count is worth
that or some automagic tuning of the batch size depending on how
effectively the batch is used. Certainly a lot of room for
experimenting.

-- 
Michal Hocko
SUSE Labs