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 19:27:59 +0000 [thread overview]
Message-ID: <220a7b090d27ffc8f3d00253c289ddd964a8462b.camel@infradead.org> (raw)
In-Reply-To: <20201027190919.GO2628@hirez.programming.kicks-ass.net>
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On Tue, 2020-10-27 at 20:09 +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2020 at 02:39:43PM +0000, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@amazon.co.uk>
> >
> > This allows an exclusive wait_queue_entry to be added at the head of the
> > queue, instead of the tail as normal. Thus, it gets to consume events
> > first without allowing non-exclusive waiters to be woken at all.
> >
> > The (first) intended use is for KVM IRQFD, which currently has
>
> Do you have more? You could easily special case this inside the KVM
> code.
I don't have more right now. What is the easy special case that you
see?
> I don't _think_ the other users of __add_wait_queue() will mind the
> extra branch, but what do I know.
I suppose we could add an unlikely() in there. It seemed like premature
optimisation.
> > static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
> > {
> > - list_add(&wq_entry->entry, &wq_head->head);
> > + struct list_head *head = &wq_head->head;
> > + struct wait_queue_entry *wq;
> > +
> > + list_for_each_entry(wq, &wq_head->head, entry) {
> > + if (!(wq->flags & WQ_FLAG_PRIORITY))
> > + break;
> > + head = &wq->entry;
> > + }
> > + list_add(&wq_entry->entry, head);
> > }
>
> So you're adding the PRIORITY things to the head of the list and need
> the PRIORITY flag to keep them in FIFO order there, right?
No, I don't care about the order of priority entries; there will
typically be only one of them; that's the point. (I'd have used the
word 'exclusive' if that wasn't already in use for something that...
well... isn't.)
I only case that the priority entries come *before* the bog-standard
non-exclusive entries (like ep_poll_callback).
The priority items end up getting added in FIFO order purely by chance,
because it was simpler to use the same insertion flow for both priority
and normal non-exclusive entries instead of making a new case. So they
all get inserted behind any existing priority entries.
> 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.
> 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.
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next prev parent reply other threads:[~2020-10-27 19:29 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 [this message]
2020-10-27 20:30 ` Peter Zijlstra
2020-10-27 20:49 ` David Woodhouse
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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