linux-kernel - Re: [RFC PATCH] mm: memcg: Do not count memory.low reclaim if it does not happen

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Message-Id: <5049EBC3-5BAE-4509-BA63-1F4A7D913517@linux.dev>
Date:   Thu, 24 Mar 2022 11:17:14 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Richard Palethorpe <rpalethorpe@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Muchun Song <songmuchun@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Yang Shi <shy828301@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tejun Heo <tj@...nel.org>, Chris Down <chris@...isdown.name>
Subject: Re: [RFC PATCH] mm: memcg: Do not count memory.low reclaim if it does not happen


> On Mar 24, 2022, at 2:52 AM, Michal Koutný <mkoutny@...e.com> wrote:
> 
> On Wed, Mar 23, 2022 at 02:44:24PM -0700, Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>> Does it mean that in the following configuration:
>>    `parent .low=50M
>>      ` s1    .low=0M   .current=50M
>>      ` s2  .low=0M   .current=50M
>> there will be no memory.events::low at all? (assuming the recursive thing is on)
> 
> True, no memory.events:low among siblings.
> Number of memory.events:low in the parent depends on how much has to be
> reclaimed (>50M means carving into parent's protection, hence it'll be
> counted).

Ok, so it’s not really about the implementation details of the reclaim mechanism (I mean rounding up to the batch size etc), it’s a more generic change: do not generate low events for cgroups not explicitly protected by a non-zero memory.low value.

Idk, I don’t have a strong argument against this change (except that it changes the existing behavior), but I also don’t see why such events are harmful. Do you mind elaborating a bit more?

Thank you!