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

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Date:	Mon, 17 Aug 2015 22:31:07 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...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>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On Fri, 7 Aug 2015, Peter Zijlstra wrote:
> On Fri, Aug 07, 2015 at 12:57:38PM +0200, Peter Zijlstra wrote:
> 
> > > >+void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
> 
> > > this one has no users the __ suggests that it is locked edition. Maybe
> > > it is for the completions…
> > 
> > Yeah, who knows, I certainly do not anymore ;-)
> 
> 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.

Well, completions are not the ones which have a gazillion of waiters
and we got quite some performance back from using swait in completions
on RT.

Thanks,

	tglx