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

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Date:	Sat, 17 Jan 2015 08:57:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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, Jan 16, 2015 at 11:48:46AM -0500, Steven Rostedt wrote:
> I notice everywhere you have a swait_wake_interruptible() but here. Is
> there a reason why?
> 
> IIRC, Peter wants to make swait wakeup usage homogenous. That is, you
> either sleep in an interruptible state, or you don't. You can't mix and
> match it.
> 
> Peter is that what you plan?

Yes, this is required for the single wakeup case to be bounded.

If you have both INTERRUPTIBLE and UNINTERRUPTIBLE waiters on a list,
and you were allowed to do INTERRUPTIBLE/UNINTERRUPTIBLE wakeups, the
wakeup would have to do O(n) iteration to find a matching waiter.

Seeing that the entire purpose of simple wait queues was to retain RT
properties, that's entirely retarded ;-)
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