Re: [Xen-devel] [PATCH v16 13/14] pvqspinlock: Improve slowpath performance by avoiding cmpxchg

[Waiman Long <waiman.long@xxxxxx>]

On 05/04/2015 10:05 AM, Peter Zijlstra wrote:
> On Thu, Apr 30, 2015 at 02:49:26PM -0400, Waiman Long wrote:
> > On 04/29/2015 02:11 PM, Peter Zijlstra wrote:
> > > On Fri, Apr 24, 2015 at 02:56:42PM -0400, Waiman Long wrote:
> > > > In the pv_scan_next() function, the slow cmpxchg atomic operation is
> > > > performed even if the other CPU is not even close to being halted. This
> > > > extra cmpxchg can harm slowpath performance.
> > > >
> > > > This patch introduces the new mayhalt flag to indicate if the other
> > > > spinning CPU is close to being halted or not. The current threshold
> > > > for x86 is 2k cpu_relax() calls. If this flag is not set, the other
> > > > spinning CPU will have at least 2k more cpu_relax() calls before
> > > > it can enter the halt state. This should give enough time for the
> > > > setting of the locked flag in struct mcs_spinlock to propagate to
> > > > that CPU without using atomic op.
> > > Yuck! I'm not at all sure you can make assumptions like that. And the
> > > worst part is, if it goes wrong the borkage is subtle and painful.
> > I do think the code is OK. However, you are right that if my reasoning is
> > incorrect, the resulting bug will be really subtle.
> So I do not think its correct, imagine the fabrics used for the 4096 cpu
> SGI machine, now add some serious traffic to them. There is no saying
> your random 2k relax loop will be enough to propagate the change.
>
> Equally, another arch (this is generic code) might have starvation
> issues on its inter-cpu fabric and delay the store just long enough.
>
> The thing is, one should _never_ rely on timing for correctness, _ever_.
Yes, you are right. Having a dependency on timing can be dangerous.

> > So I am going to
> > withdraw this particular patch as it has no functional impact to the overall
> > patch series. Please let me know if you have any other comments on other
> > parts of the series and I will send send out a new series without this
> > particular patch.
> Please wait a little while, I've queued the 'basic' patches, once that
> settles in tip we can look at the others.
>
> Also, I have some local changes (sorry, I could not help mysef) I should
> post, I've been somewhat delayed by illness.
Sure. I will wait until you finish your tip test.

I am sorry to hear that you are bothered with illness. I hope you get 
well by now.
Cheers,
Longman

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