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Message-ID: <c3875745-dae6-4b79-8c4f-86b4d7147db9@kernel.org>
Date: Thu, 27 Jun 2024 21:18:35 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>, Yosry Ahmed <yosryahmed@...gle.com>
Cc: tj@...nel.org, cgroups@...r.kernel.org, hannes@...xchg.org,
lizefan.x@...edance.com, longman@...hat.com, kernel-team@...udflare.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 2/2] cgroup/rstat: Avoid thundering herd problem by
kswapd across NUMA nodes
On 27/06/2024 20.45, Shakeel Butt wrote:
> On Thu, Jun 27, 2024 at 04:32:03AM GMT, Yosry Ahmed wrote:
>> On Thu, Jun 27, 2024 at 3:33 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> [...]
>>>
>>> The reason why I suggested that the completion live in struct cgroup
>>> is because there is a chance here that the flush completes and another
>>> irrelevant flush starts between reading cgrp_rstat_ongoing_flusher and
>>> calling wait_for_completion_interruptible_timeout().
>
I didn't add this per cgroup because I fear the race of adding a
wait_for_completion on a cgroup that gets stuck there, but looking at
the code the completion API should be able to avoid this.
> Yes this can happen if flusher for irrelevant cgroup calls
> reinit_completion() while the initial flusher was just about to call
> wait_for_completion_interruptible_timeout().
>
Restoring two main functions to assist reviewer seeing the race:
On 26/06/2024 23.18, Jesper Dangaard Brouer wrote:
> +#define MAX_WAIT msecs_to_jiffies(100)
> +/* Trylock helper that also checks for on ongoing flusher */
> +static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp)
> +{
> +retry:
> + bool locked = __cgroup_rstat_trylock(cgrp, -1);
> + if (!locked) {
> + struct cgroup *cgrp_ongoing;
> +
> + /* Lock is contended, lets check if ongoing flusher is
> + * taking care of this, if we are a descendant.
> + */
> + cgrp_ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
> + if (!cgrp_ongoing)
> + goto retry;
> +
Long wait/race here, can cause us to see an out-dated cgrp_ongoing.
And then another CPU manage to reach reinit_completion() below, before
execution continues here.
> + if (cgroup_is_descendant(cgrp, cgrp_ongoing)) {
> + wait_for_completion_interruptible_timeout(
> + &cgrp_rstat_flusher_done, MAX_WAIT);
> +
> + return false;
> + }
> + __cgroup_rstat_lock(cgrp, -1, false);
> + }
> + /* Obtained lock, record this cgrp as the ongoing flusher */
> + reinit_completion(&cgrp_rstat_flusher_done);
> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp);
> +
> + return true; /* locked */
> +}
> +
> +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp)
> +{
> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL);
> + complete_all(&cgrp_rstat_flusher_done);
> + __cgroup_rstat_unlock(cgrp, -1);
> +}
>>>
>>> This will cause the caller to wait for an irrelevant flush, which may
>>> be fine because today the caller would wait for the lock anyway. Just
>>> mentioning this in case you think this may happen enough to be a
>>> problem.
Yes, it would wait for an irrelevant flush.
>>
>> Actually, I think this can happen beyond the window I described above.
>> I think it's possible that a thread waits for the flush, then gets
>> woken up when complete_all() is called, but another flusher calls
>> reinit_completion() immediately. The woken up thread will observe
>> completion->done == 0 and go to sleep again.
>
> I don't think it will go to sleep again as there is no retry.
>
>>
>> I think most of these cases can be avoided if we make the completion
>> per cgroup. It is still possible to wait for more flushes than
>> necessary, but only if they are for the same cgroup.
>
> Yeah, per-cgroup completion would avoid the problem of waiting for
> irrelevant flush.
Great, I will code up a version with per-cgroup completion.
--Jesper
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