linux-kernel - Re: Expensive memory.stat + cpu.stat reads

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Message-ID: <ZNrEV1qEcQMUgztn@slm.duckdns.org>
Date:   Mon, 14 Aug 2023 14:18:31 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Ivan Babrou <ivan@...udflare.com>,
        Waiman Long <longman@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>, cgroups@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        kernel-team <kernel-team@...udflare.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Expensive memory.stat + cpu.stat reads

Hello,

On Fri, Aug 11, 2023 at 05:01:08PM -0700, Yosry Ahmed wrote:
> There have been a lot of problems coming from this global rstat lock:
> hard lockups (when we used to flush atomically), unified flushing
> being expensive, skipping flushing being inaccurate, etc.
> 
> I wonder if it's time to rethink this lock and break it down into
> granular locks. Perhaps a per-cgroup lock, and develop a locking
> scheme where you always lock a parent then a child, then flush the
> child and unlock it and move to the next child, etc. This will allow
> concurrent flushing of non-root cgroups. Even when flushing the root,
> if we flush all its children first without locking the root, then only
> lock the root when flushing the top-level children, then some level of
> concurrency can be achieved.
> 
> Maybe this is too complicated, I never tried to implement it, but I
> have been bouncing around this idea in my head for a while now.
> 
> We can also split the update tree per controller. As far as I can tell
> there is no reason to flush cpu stats for example when someone wants
> to read memory stats.

There's another thread. Let's continue there but I'm a bit skeptical whether
splitting the lock is a good solution here. Regardless of locking, we don't
want to run in an atomic context for that long anwyay.

Thanks.

-- 
tejun