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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 27 Jul 2021 09:51:00 -0700
Message-ID: <CALvZod5JmcSpym6oXMXEUXEq2u13SVLRaVGPg84ubeYZt-6w_g@mail.gmail.com>
Subject: Re: [PATCH] mm: memcontrol: fix blocking rstat function called from
 atomic cgroup1 thresholding code
To:     Rik van Riel <riel@fb.com>
Cc:     "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Kernel Team <Kernel-team@fb.com>,
        "dan.carpenter@oracle.com" <dan.carpenter@oracle.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>
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On Mon, Jul 26, 2021 at 8:19 AM Rik van Riel <riel@fb.com> wrote:
>
> On Mon, 2021-07-26 at 11:00 -0400, Johannes Weiner wrote:
> >
> > __mem_cgroup_threshold() indeed holds the rcu lock. In addition, the
> > thresholding code is invoked during stat changes, and those contexts
> > have irqs disabled as well. If the lock breaking occurs inside the
> > flush function, it will result in a sleep from an atomic context.
> >
> > Use the irsafe flushing variant in mem_cgroup_usage() to fix this
>
> While this fix is necessary, in the long term I think we may
> want some sort of redesign here, to make sure the irq safe
> version does not spin long times trying to get the statistics
> off some other CPU.
>
> I have seen a number of soft (IIRC) lockups deep inside the
> bowels of cgroup_rstat_flush_irqsafe, with the function taking
> multiple seconds to complete.

Can you please share a bit more detail on this lockup? I am wondering
if this was due to the flush not happening more often and thus the
update tree is large or if there are too many concurrent flushes
happening.

>
> Reviewed-by: Rik van Riel <riel@surriel.com>