linux-kernel - Re: [RESEND RFC PATCH] epoll: autoremove wakers even more aggressively

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Date:   Fri, 15 Jul 2022 21:55:28 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Benjamin Segall <bsegall@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Roman Penyaev <rpenyaev@...e.de>,
        Jason Baron <jbaron@...mai.com>,
        Khazhismel Kumykov <khazhy@...gle.com>, Heiher <r@....cc>
Subject: Re: [RESEND RFC PATCH] epoll: autoremove wakers even more
 aggressively

On Sat, 16 Jul 2022 01:27:31 +0000 Shakeel Butt <shakeelb@...gle.com> wrote:

>
> ...
>
> > > > production with real workloads and it has caused hard lockups.
> > > > Particularly network heavy workloads with a lot of threads in
> > > > epoll_wait() can easily trigger this issue if they get killed
> > > > (oom-killed in our case).
> > >
> > > Hard lockups are undesirable.  Is a cc:stable justified here?
> > 
> > Not for now as I don't know if we can blame a patch which might be the
> > source of this behavior.
> 
> I am able to repro the epoll hard lockup on next-20220715 with Ben's
> patch reverted. The repro is a simple TCP server and tens of clients
> communicating over loopback. Though to cause the hard lockup I have to
> create a couple thousand threads in epoll_wait() in server and also
> reduce the kernel.watchdog_thresh. With Ben's patch the repro does not
> cause the hard lockup even with kernel.watchdog.thresh=1.
> 
> Please add:
> 
> Tested-by: Shakeel Butt <shakeelb@...gle.com>

OK, thanks.  I added the cc:stable.  No Fixes:, as it has presumably
been there for a long time, perhaps for all time.