linux-kernel - Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

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Date:	Fri, 7 Aug 2015 18:45:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Mackerras <paulus@...ba.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On Fri, Aug 07, 2015 at 09:41:31AM -0700, Christoph Hellwig wrote:
> On Fri, Aug 07, 2015 at 01:14:15PM +0200, Peter Zijlstra wrote:
> > On that, we cannot convert completions to swait. Because swait wake_all
> > must not happen from IRQ context, and complete_all() typically is used
> > from just that.
> 
> If swait queues aren't useable from IRQ context they will be fairly
> useless.  What's the problem with making them irq safe?

Its just the swait_wake_all() that is not. The entire purpose of them
was to have something that allows bounded execution (RT and all).

Since you can have unbounded numbers of tasks waiting on a waitqueue
(well, reality has bounds of course, like total memory available etc..)
a wake_all() can end up being many many wake_process() calls.

We've had this be a problem in RT.

So the proposed swait_wake_all() requires being called from task
context, such that it can drop the lock (and IRQ disable) after every
wakeup, and thereby guarantee that higher priority things will not
experience undue latencies.
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