linux-kernel - Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue

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Date:   Tue, 27 Oct 2020 20:49:45 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()

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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