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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 28 Jul 2021 13:37:28 -0700
Message-ID: <CALvZod71oM0=B_u6-fSMFBqwP2tEOgxKxygHkcshmNF-BMCwzg@mail.gmail.com>
Subject: Re: [PATCH] memcg: cleanup racy sum avoidance code
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jul 28, 2021 at 12:43 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Tue, 27 Jul 2021 18:22:43 -0700 Shakeel Butt <shakeelb@google.com> wrote:
>
> > We used to have per-cpu memcg and lruvec stats and the readers have to
> > traverse and sum the stats from each cpu. This summing was racy and may
> > expose transient negative values. So, an explicit check was added to
> > avoid such scenarios. Now these stats are moved to rstat infrastructure
> > and are no more per-cpu, so we can remove the fixup for transient
> > negative values.
>
> We can't do anything about the same code in lruvec_page_state_local()?

lruvec_page_state_local() is used by cgroup v1's memory.numa_stat for
cgroup local stats (not hierarchical) and are still per-cpu. To make
it non-per-cpu, we have to add 'long
state_local[NR_VM_NODE_STAT_ITEMS]' in 'struct lruvec_stats' and do
aggregation in rstat flushing. So, paying the cpu traversal cost with
more memory usage. I am not sure if it is worth it.