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

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Message-ID: <20160625192025.GP30154@twins.programming.kicks-ass.net>
Date:	Sat, 25 Jun 2016 21:20:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	panxinhui <xinhui@...ux.vnet.ibm.com>
Cc:	Boqun Feng <boqun.feng@...il.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 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.