linux-kernel - Re: [PATCH 0/9] qspinlock stuff -v15

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Message-ID: <20150326202153.GD27490@worktop.programming.kicks-ass.net>
Date:	Thu, 26 Mar 2015 21:21:53 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Waiman.Long@...com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, paolo.bonzini@...il.com, boris.ostrovsky@...cle.com,
	paulmck@...ux.vnet.ibm.com, riel@...hat.com,
	torvalds@...ux-foundation.org, raghavendra.kt@...ux.vnet.ibm.com,
	david.vrabel@...rix.com, oleg@...hat.com, scott.norton@...com,
	doug.hatch@...com, linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	luto@...capital.net
Subject: Re: [PATCH 0/9] qspinlock stuff -v15

On Wed, Mar 25, 2015 at 03:47:39PM -0400, Konrad Rzeszutek Wilk wrote:
> Ah nice. That could be spun out as a seperate patch to optimize the existing
> ticket locks I presume.

Yes I suppose we can do something similar for the ticket and patch in
the right increment. We'd need to restructure the code a bit, but
its not fundamentally impossible.

We could equally apply the head hashing to the current ticket
implementation and avoid the current bitmap iteration.

> Now with the old pv ticketlock code an vCPU would only go to sleep once and
> be woken up when it was its turn. With this new code it is woken up twice 
> (and twice it goes to sleep). With an overcommit scenario this would imply
> that we will have at least twice as many VMEXIT as with the previous code.

An astute observation, I had not considered that.

> I presume when you did benchmarking this did not even register? Thought
> I wonder if it would if you ran the benchmark for a week or so.

You presume I benchmarked :-) I managed to boot something virt and run
hackbench in it. I wouldn't know a representative virt setup if I ran
into it.

The thing is, we want this qspinlock for real hardware because its
faster and I really want to avoid having to carry two spinlock
implementations -- although I suppose that if we really really have to
we could.
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