linux-kernel - Re: [PATCH 2/4] selftests: memcg: Expect no low events in unprotected sibling

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Date:   Wed, 18 May 2022 17:54:44 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     void@...ifault.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, hannes@...xchg.org, kernel-team@...com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mhocko@...nel.org, shakeelb@...gle.com, tj@...nel.org,
        Richard Palethorpe <rpalethorpe@...e.de>
Subject: Re: [PATCH 2/4] selftests: memcg: Expect no low events in
 unprotected sibling

Hi.

On Fri, May 13, 2022 at 11:54:16AM -0700, Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> Hm, what are our plans here? Are we going to fix it soon-ish, or there
> is still no agreement on how to proceed?

Here are some of my ideas in random order so far and comments:

0) mask memory.events:low
-> not a real fix

1) don't do SWAP_CLUSTER_MAX roundup
-> won't solve sudden lift of protection

2) instead of SWAP_CLUSTER_MAX over-reclaim, do same under-reclaim
-> same problem as 1)

3) update children_low_usage transactionally (after reclaim round is done)
- ???

4) don't recursively distribute residual protection in the same reclaim round
- ???

5) iterate siblings from highest to lowest protection 
- not a solution

6) assign only >SWAP_CLUSTER_MAX of residuum
- need more info

I'm discouraged by possible complexity of 3) or 4), while 6) is what I'd
like to look more into.

HTH,
Michal