linux-kernel - Re: [RFC PATCH] mm: silence soft lockups from unlock

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Date:   Sun, 26 Jul 2020 15:57:05 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page

Linus,

I was greatly confused and tried to confuse you.

Somehow I misunderstood your last version and didn't bother to read it
again until now.

Sorry for noise and thanks for your explanations.

Oleg.


On 07/25, Linus Torvalds wrote:
>
> On Sat, Jul 25, 2020 at 12:28 PM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > What I tried to say. AFAICS before that commit we had (almost) the same
> > behaviour you propose now: unlock_page/etc wakes all the non-exclusive
> > waiters up.
> >
> > No?
>
> Yes, but no.
>
> We'd wake them _up_ fairly aggressively, but then they'd be caught on
> the bit being set again by the exclusive locker (that we also woke
> up).
>
> So they'd get woken up, and then go to sleep again.
>
> So the new behavior wakes things up more aggressively (but a different
> way), but not by letting them go out of order and early, but simply by
> not going back to sleep again.
>
> So the "wake up more" is very different - now it's about not going to
> sleep again, rather than by ordering the wakeup queue.
>
> We _could_ order the wakeup queue too, and put all non-exclusive
> weiters at the head again. And make it *really* aggressive.
>
> But since one of ourissues has been "latency of walking the wait
> queue", I'm not sure we want that. interspesing any blocking waiters -
> and stopping the waitqueue walking as a result - might be better under
> load.
>
> Wild handwaving. We could try it, but IO think that really would be a
> separate "try this out" patch.
>
> Right now, I think my patch will likely make for _better_ latencies
> for everything.
>
> Lower latency of non-exclusive waiters (because not going back to
> sleep), but also lower latency of walking the wait queue (because
> fewer entries, hopefully, and also less contention due to the "not
> going back to sleep" noise)
>
>            Linus
>