linux-kernel - Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning

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Message-ID: <20150916150307.GD3604@twins.programming.kicks-ass.net>
Date:	Wed, 16 Sep 2015 17:03:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning

On Tue, Sep 15, 2015 at 11:32:14AM -0400, Waiman Long wrote:
> On 09/15/2015 04:38 AM, Peter Zijlstra wrote:
> >On Mon, Sep 14, 2015 at 03:37:32PM -0400, Waiman Long wrote:
> >>BTW, the queue head vCPU at pv_wait_head_and_lock() doesn't wait early, it
> >>will spin the full threshold as there is no way for it to figure out if the
> >>lock holder is running or not.
> >We can know its cpu id, right? Surely we should then be able to figure
> >out if its active or not, I'm thinking the KVM/Xen know these things
> >somewhere.
> 
> We can make a guess of the lock holder cpu id by peeking at previous MCS
> node. However, if the current lock holder got the lock without entering the
> queue or it got the lock by stealing, we won't have information of what CPU
> it is.

Ah, yes I see, we do not have storage for the lock holder. Yes you're
right.
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