linux-kernel - Re: [PATCH] locking/osq: Drop the overload of osq lock

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Date:	Sun, 26 Jun 2016 10:26:32 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	panxinhui <xinhui@...ux.vnet.ibm.com>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, dave@...olabs.net,
	will.deacon@....com, Waiman.Long@....com, benh@...nel.crashing.org
Subject: Re: [PATCH] locking/osq: Drop the overload of osq lock

On Sat, Jun 25, 2016 at 09:20:25PM +0200, Peter Zijlstra wrote:
> On Sun, Jun 26, 2016 at 01:27:56AM +0800, panxinhui wrote:
> > >> Would that not have issues where the owner cpu is kept running but the
> > >> spinner (ie. _this_ vcpu) gets preempted? I would think that in that
> > >> case we too want to stop spinning.
> > >> 
> > > 
> > do  you mean that the spinner detect itself had yield out during the
> > big spin loop?
> > 
> > It is very possible to happen.  BUT if spinner(on this vcpu) yield
> > out, the next spinner would break the spin loop.  AND if spinner
> > detect itself yield out once, it’s very possible to get the osq lock
> > soon as long as the ower vcpu is running.
> > 
> > SO I think we need just check the owner vcpu’s yield_count.
> 
> I had a quick look at KVM and it looks like it only has
> kvm_cpu::preempted, which would suggest the interface boqun proposed.
> 
> We'll have to look at many of the other virt platforms as well to see
> what they can do.
> 
> We could also provide _both_ interfaces and a platform can implement
> whichever variant (or both) it can.
> 

Make sense ;-)

Lemme cook something for further discussions.

Regards,
Boqun

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