From: Michal Hocko <mhocko@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page
Date: Tue, 21 Jul 2020 17:49:39 +0200 [thread overview]
Message-ID: <20200721154939.GO4061@dhcp22.suse.cz> (raw)
In-Reply-To: <CAHk-=whewL14RgwLZTXcNAnrDPt0H+sRJS6iDq0oGb6zwaBMxg@mail.gmail.com>
On Tue 21-07-20 08:33:33, Linus Torvalds wrote:
> On Mon, Jul 20, 2020 at 11:33 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > The lockup is in page_unlock in do_read_fault and I suspect that this is
> > yet another effect of a very long waitqueue chain which has been
> > addresses by 11a19c7b099f ("sched/wait: Introduce wakeup boomark in
> > wake_up_page_bit") previously.
>
> Hmm.
>
> I do not believe that you can actually get to the point where you have
> a million waiters and it takes 20+ seconds to wake everybody up.
I was really suprised as well!
> More likely, it's actually *caused* by that commit 11a19c7b099f, and
> what might be happening is that other CPU's are just adding new
> waiters to the list *while* we're waking things up, because somebody
> else already got the page lock again.
>
> Humor me.. Does something like this work instead? It's
> whitespace-damaged because of just a cut-and-paste, but it's entirely
> untested, and I haven't really thought about any memory ordering
> issues, but I think it's ok.
>
> The logic is that anybody who called wake_up_page_bit() _must_ have
> cleared that bit before that. So if we ever see it set again (and
> memory ordering doesn't matter), then clearly somebody else got access
> to the page bit (whichever it was), and we should not
>
> (a) waste time waking up people who can't get the bit anyway
>
> (b) be in a livelock where other CPU's continually add themselves to
> the wait queue because somebody else got the bit.
>
> and it's that (b) case that I think happens for you.
>
> NOTE! Totally UNTESTED patch follows. I think it's good, but maybe
> somebody sees some problem with this approach?
I can ask them to give it a try.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-07-21 15:50 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-21 6:32 [RFC PATCH] mm: silence soft lockups from unlock_page Michal Hocko
[not found] ` <FCC3EB2D-9F11-4E9E-88F4-40B2926B35CC@lca.pw>
2020-07-21 11:25 ` Michal Hocko
[not found] ` <664A07B6-DBCD-4520-84F1-241A4E7A339F@lca.pw>
2020-07-21 12:17 ` Michal Hocko
[not found] ` <20200721132343.GA4261@lca.pw>
2020-07-21 13:38 ` Michal Hocko
2020-07-21 14:17 ` Chris Down
2020-07-21 15:00 ` Michal Hocko
2020-07-21 15:33 ` Linus Torvalds
2020-07-21 15:49 ` Michal Hocko [this message]
2020-07-22 18:29 ` Linus Torvalds
2020-07-22 21:29 ` Hugh Dickins
2020-07-22 22:10 ` Linus Torvalds
2020-07-22 23:42 ` Linus Torvalds
2020-07-23 0:23 ` Linus Torvalds
2020-07-23 12:47 ` Oleg Nesterov
2020-07-23 17:32 ` Linus Torvalds
2020-07-23 18:01 ` Oleg Nesterov
2020-07-23 18:22 ` Linus Torvalds
2020-07-23 19:03 ` Linus Torvalds
2020-07-24 14:45 ` Oleg Nesterov
2020-07-23 20:03 ` Linus Torvalds
2020-07-23 23:11 ` Hugh Dickins
2020-07-23 23:43 ` Linus Torvalds
2020-07-24 0:07 ` Hugh Dickins
2020-07-24 0:46 ` Linus Torvalds
2020-07-24 3:45 ` Hugh Dickins
2020-07-24 15:24 ` Oleg Nesterov
2020-07-24 17:32 ` Linus Torvalds
2020-07-24 23:25 ` Linus Torvalds
2020-07-25 2:08 ` Hugh Dickins
2020-07-25 2:46 ` Linus Torvalds
2020-07-25 10:14 ` Oleg Nesterov
2020-07-25 18:48 ` Linus Torvalds
2020-07-25 19:27 ` Oleg Nesterov
2020-07-25 19:51 ` Linus Torvalds
2020-07-26 13:57 ` Oleg Nesterov
2020-07-25 21:19 ` Hugh Dickins
2020-07-26 4:22 ` Hugh Dickins
2020-07-26 20:30 ` Hugh Dickins
2020-07-26 20:41 ` Linus Torvalds
2020-07-26 22:09 ` Hugh Dickins
2020-07-27 19:35 ` Greg KH
2020-08-06 5:46 ` Hugh Dickins
2020-08-18 13:50 ` Greg KH
2020-08-06 5:21 ` Hugh Dickins
2020-08-06 17:07 ` Linus Torvalds
2020-08-06 18:00 ` Matthew Wilcox
2020-08-06 18:32 ` Linus Torvalds
2020-08-07 18:41 ` Hugh Dickins
2020-08-07 19:07 ` Linus Torvalds
2020-08-07 19:35 ` Matthew Wilcox
2020-08-03 13:14 ` Michal Hocko
2020-08-03 17:56 ` Linus Torvalds
2020-07-25 9:39 ` Oleg Nesterov
2020-07-23 8:03 ` Michal Hocko
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