linux-kernel - Re: [PATCH] mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code

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Date:   Tue, 3 Aug 2021 14:34:23 +0000
From:   Rik van Riel <riel@...com>
To:     "shakeelb@...gle.com" <shakeelb@...gle.com>
CC:     Kernel Team <Kernel-team@...com>,
        "dan.carpenter@...cle.com" <dan.carpenter@...cle.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: memcontrol: fix blocking rstat function called from
 atomic cgroup1 thresholding code

On Tue, 2021-07-27 at 09:51 -0700, Shakeel Butt wrote:
> On Mon, Jul 26, 2021 at 8:19 AM Rik van Riel <riel@...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.

I was not logged into any system while it happened, but
only found it later in the logs.

I suspect your explanation is the reason why it happened,
though.