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Date: Tue, 15 Sep 2015 11:32:14 -0400
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning
References: <1441996658-62854-1-git-send-email-Waiman.Long@hpe.com> <1441996658-62854-7-git-send-email-Waiman.Long@hpe.com> <20150914141012.GV18489@twins.programming.kicks-ass.net> <55F721FC.4000504@hpe.com> <20150915083804.GY16853@twins.programming.kicks-ass.net>
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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.

Cheers,
Longman