linux-kernel - Re: [PATCH 0/6] Drain remote per-cpu directly v3

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Date:   Thu, 19 May 2022 17:05:04 -0400
From:   Qian Cai <quic_qiancai@...cinc.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
CC:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, <kafai@...com>,
        <kpsingh@...nel.org>
Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3

On Thu, May 19, 2022 at 12:15:24PM -0700, Paul E. McKenney wrote:
> Is the task doing offline_pages()->synchronize_rcu() doing this
> repeatedly?  Or is there a stalled RCU grace period?  (From what
> I can see, offline_pages() is not doing huge numbers of calls to
> synchronize_rcu() in any of its loops, but I freely admit that I do not
> know this code.)

Yes, we are running into an endless loop in isolate_single_pageblock().
There was a similar issue happened not long ago, so I am wondering if we
did not solve it entirely then. Anyway, I will continue the thread over
there.

https://lore.kernel.org/all/YoavU%2F+NfQIzQiDF@qian/

> Or is it possible that reverting those three patches simply decreases
> the probability of failure, rather than eliminating the failure?
> Such a decrease could be due to many things, for example, changes to
> offsets and sizes of data structures.

Entirely possible. Sorry for the false alarm.

> Do you ever see RCU CPU stall warnings?

No.