linux-kernel - Re: [PATCH v2] powerpc ticket locks

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Date:	Wed, 12 Feb 2014 06:34:28 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Torsten Duwe <duwe@....de>,
	Raghavendra KT <raghavendra.kt.linux@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tom Musta <tommusta@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] powerpc ticket locks

On Tue, 2014-02-11 at 12:30 -0600, Scott Wood wrote:
> > It's something very special to IBM pSeries: the hypervisor can assign
> > fractions of physical CPUs to guests. Sometimes a guest with 4 quarter
> > CPUs will be faster than 1 monoprocessor. (correct me if I'm wrong).
> > 
> > The directed yield resolves the silly situation when holder and waiter
> > reside on the same physical CPU, as I understand it.
> > 
> > x86 has nothing comparable.
> 
> How is this different from the very ordinary case of an SMP KVM guest
> whose vcpus are not bound to host cpus, and thus you could have multiple
> vcpus running on the same host cpu?

It's not really ... though I can see drawbacks with the scheme as well
and I think in KVM we should be careful to only confer if the owner
vcpu last scheduled on the same physical cpu where the waiter is, other
wise, there's too much chances of us bouncing things around the machine
for minor contention cases.

Paul, what's your policy today ?

Cheers,
Ben.


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