From: David Woodhouse <dwmw2@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()
Date: Tue, 27 Oct 2020 20:49:45 +0000 [thread overview]
Message-ID: <1e10666c086c0f5221d399de974fcff5f05e61b4.camel@infradead.org> (raw)
In-Reply-To: <20201027203041.GS2628@hirez.programming.kicks-ass.net>
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On Tue, 2020-10-27 at 21:30 +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2020 at 07:27:59PM +0000, David Woodhouse wrote:
>
> > > While looking at this I found that weird __add_wait_queue_exclusive()
> > > which is used by fs/eventpoll.c and does something similar, except it
> > > doesn't keep the FIFO order.
> >
> > It does, doesn't it? Except those so-called "exclusive" entries end up
> > in FIFO order amongst themselves at the *tail* of the queue, to be
> > woken up only after all the other entries before them *haven't* been
> > excluded.
>
> __add_wait_queue_exclusive() uses __add_wait_queue() which does
> list_add(). It does _not_ add at the tail like normal exclusive users,
> and there is exactly _1_ user in tree that does this.
>
> I'm not exactly sure how this happened, but:
>
> add_wait_queue_exclusive()
>
> and
>
> __add_wait_queue_exclusive()
>
> are not related :-(
Oh, that is *particularly* special.
It sounds like the __add_wait_queue_exclusive() version is a half-baked
attempt at doing what I'm doing here, except....
> > > The Changelog doesn't state how important this property is to you.
> >
> > Because it isn't :)
> >
> > The ordering is:
> >
> > { PRIORITY }* { NON-EXCLUSIVE }* { EXCLUSIVE(sic) }*
> >
> > I care that PRIORITY comes before the others, because I want to
> > actually exclude the others. Especially the "non-exclusive" ones, which
> > the 'exclusive' ones don't actually exclude.
> >
> > I absolutely don't care about ordering *within* the set of PRIORITY
> > entries, since as I said I expect there to be only one.
>
> Then you could arguably do something like:
>
> spin_lock_irqsave(&wq_head->lock, flags);
> __add_wait_queue_exclusive(wq_head, wq_entry);
> spin_unlock_irqrestore(&wq_head->lock, flags);
>
> and leave it at that.
.. the problem with that is that other waiters *can* end up on the
queue before it, if they are added later. I don't know if the existing
user (ep_poll) cares, but I do.
> But now I'm itching to fix that horrible naming... tomorrow perhaps.
:)
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next prev parent reply other threads:[~2020-10-27 20:50 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-26 17:53 [RFC PATCH 1/2] sched/wait: Add add_wait_queue_priority() David Woodhouse
2020-10-26 17:53 ` [RFC PATCH 2/2] kvm/eventfd: Use priority waitqueue to catch events before userspace David Woodhouse
2020-10-27 8:01 ` Paolo Bonzini
2020-10-27 10:15 ` David Woodhouse
2020-10-27 13:55 ` [PATCH 0/3] Allow in-kernel consumers to drain events from eventfd David Woodhouse
2020-10-27 13:55 ` [PATCH 1/3] eventfd: Export eventfd_ctx_do_read() David Woodhouse
2020-10-27 13:55 ` [PATCH 2/3] vfio/virqfd: Drain events from eventfd in virqfd_wakeup() David Woodhouse
2020-11-06 23:29 ` Alex Williamson
2020-11-08 9:17 ` Paolo Bonzini
2020-10-27 13:55 ` [PATCH 3/3] kvm/eventfd: Drain events from eventfd in irqfd_wakeup() David Woodhouse
2020-10-27 18:41 ` kernel test robot
2020-10-27 21:42 ` kernel test robot
2020-10-27 23:13 ` kernel test robot
2020-10-27 14:39 ` [PATCH v2 0/2] Allow KVM IRQFD to consistently intercept events David Woodhouse
2020-10-27 14:39 ` [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority() David Woodhouse
2020-10-27 19:09 ` Peter Zijlstra
2020-10-27 19:27 ` David Woodhouse
2020-10-27 20:30 ` Peter Zijlstra
2020-10-27 20:49 ` David Woodhouse [this message]
2020-10-27 21:32 ` David Woodhouse
2020-10-28 14:20 ` Peter Zijlstra
2020-10-28 14:44 ` Paolo Bonzini
2020-10-28 14:35 ` Peter Zijlstra
2020-11-04 9:35 ` David Woodhouse
2020-11-04 11:25 ` Paolo Bonzini
2020-11-06 10:17 ` Paolo Bonzini
2020-11-06 16:32 ` Alex Williamson
2020-11-06 17:18 ` David Woodhouse
2020-10-27 14:39 ` [PATCH v2 2/2] kvm/eventfd: Use priority waitqueue to catch events before userspace David Woodhouse
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