Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt
Date	2015-10-14 11:30 +0200
Message-ID	<qjqxI-2DR-15@gated-at.bofh.it> (permalink)
References	<qbCPn-8bs-3@gated-at.bofh.it> <qbCPo-8bs-7@gated-at.bofh.it> <qjdTR-8ds-39@gated-at.bofh.it> <qjePV-18U-21@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Oct 13, 2015 at 04:50:25PM -0400, Waiman Long wrote:
> On 10/13/2015 03:56 PM, Peter Zijlstra wrote:

> >So the below is exactly duplicated from the normal slowpath, so why
> >don't you keep that there?
> >
> >It would get you something like:
> >
> >	if (pv_wait_head_or_steal(..))
> >		goto stolen;
> >
> >
> >stolen:
> >>+	/*
> >>+	 * contended path; wait for next, release.
> >>+	 */
> >>+	while (!(next = READ_ONCE(node->next)))
> >>+		cpu_relax();
> >>+
> >>+	arch_mcs_spin_unlock_contended(&next->locked);
> >>+	pv_kick_node(lock, next);
> >release:
> >	...
> 
> Yes, it is largely the same. I thought that you don't like too much change
> in the logic flow of the generic qspinlock code. I will make the change in
> the next revision.

Well, you already put the branch in there, the only difference here is
an 'extra' label. OTOH that extra label avoids duplicating some hairy
code. So over all I would say its a definite win.

And its easy to see it will compile away on the native case where:

 #define pv_wait_head_or_steal(l, n, t) (false)


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Thread

Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt Peter Zijlstra <peterz@infradead.org> - 2015-10-13 22:00 +0200
  Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt Waiman Long <waiman.long@hpe.com> - 2015-10-13 23:00 +0200
    Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt Peter Zijlstra <peterz@infradead.org> - 2015-10-14 11:30 +0200
      Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt Waiman Long <waiman.long@hpe.com> - 2015-10-15 23:10 +0200

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