linux-kernel - Re: [PATCH] mm: memcg: make sure memory.events is uptodate when waking pollers

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Message-ID: <20180405194512.GD3126663@devbig577.frc2.facebook.com>
Date:   Thu, 5 Apr 2018 12:45:12 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Michal Hocko <mhocko@...nel.org>, vdavydov.dev@...il.com,
        guro@...com, riel@...riel.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: memcg: make sure memory.events is uptodate when
 waking pollers

On Thu, Apr 05, 2018 at 01:55:16PM -0400, Johannes Weiner wrote:
> From 4369ce161a9085aa408f2eca54f9de72909ee1b1 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Thu, 5 Apr 2018 11:53:55 -0400
> Subject: [PATCH] mm: memcg: make sure memory.events is uptodate when waking
>  pollers
> 
> a983b5ebee57 ("mm: memcontrol: fix excessive complexity in memory.stat
> reporting") added per-cpu drift to all memory cgroup stats and events
> shown in memory.stat and memory.events.
> 
> For memory.stat this is acceptable. But memory.events issues file
> notifications, and somebody polling the file for changes will be
> confused when the counters in it are unchanged after a wakeup.
> 
> Luckily, the events in memory.events - MEMCG_LOW, MEMCG_HIGH,
> MEMCG_MAX, MEMCG_OOM - are sufficiently rare and high-level that we
> don't need per-cpu buffering for them: MEMCG_HIGH and MEMCG_MAX would
> be the most frequent, but they're counting invocations of reclaim,
> which is a complex operation that touches many shared cachelines.
> 
> This splits memory.events from the generic VM events and tracks them
> in their own, unbuffered atomic counters. That's also cleaner, as it
> eliminates the ugly enum nesting of VM and cgroup events.
> 
> Fixes: a983b5ebee57 ("mm: memcontrol: fix excessive complexity in memory.stat reporting")
> Reported-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Yeah, that works.  FWIW,

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun