linux-kernel - Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude protection

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Date:   Fri, 22 Mar 2019 22:49:46 +0000
From:   Chris Down <chris@...isdown.name>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Dennis Zhou <dennis@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude
 protection

Roman Gushchin writes:
>I've noticed that the old version is just wrong: if cgroup_size is way smaller
>than max(min, low), scan will be set to -lruvec_size.
>Given that it's unsigned long, we'll end up with scanning the whole list
>(due to clamp() below).

Are you certain? If so, I don't see what you mean. This is how the code looks 
in Linus' tree after the fixups:

    unsigned long cgroup_size = mem_cgroup_size(memcg);
    unsigned long baseline = 0;

    if (!sc->memcg_low_reclaim)
            baseline = lruvec_size;
    scan = lruvec_size * cgroup_size / protection - baseline;

This works correctly as far as I can tell:

low reclaim case:

    In [1]: cgroup_size=50; lruvec_size=10; protection=2000; baseline=0; lruvec_size * cgroup_size // protection - baseline
    Out[1]: 0

normal case:

    In [2]: cgroup_size=3000; lruvec_size=10; protection=2000; baseline=lruvec_size; lruvec_size * cgroup_size // protection - baseline
    Out[2]: 5