linux-kernel - Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

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Message-ID: <20150904073917.GA18489@twins.programming.kicks-ass.net>
Date:	Fri, 4 Sep 2015 09:39:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [4.2, Regression] Queued spinlocks cause major XFS performance
 regression

On Thu, Sep 03, 2015 at 11:39:21PM -0700, Linus Torvalds wrote:
> On Thu, Sep 3, 2015 at 10:48 PM, Dave Chinner <david@...morbit.com> wrote:
> >
> > When I turned spinlock debugging off on 4.2 to get some perf numbers
> > a request from Linus, I got this:

> > To confirm that this is indeed caused by the queued spinlocks, I
> > removed the the spinlock debugging and did this to arch/x86/Kconfig:
> >
> > -       select ARCH_USE_QUEUED_SPINLOCK
> >
> > And the results are:

> Is this a PARAVIRT configuration? There were issues with PV
> interaction at some point. If it is PV, and you don't actually use PV,
> can you test with PV support disabled?

His earlier instructions on how to reproduce do indeed have the workload
running inside a VM.

Dave, can you confirm you're still running this in a VM, and can you
provide your full guest .config?  In particular the PARAVIRT_SPINLOCKS
option is interesting.

Also, are you doing anything special with the host?

I'll try and poke at this, but I'm not much into this whole VM thing so
it'll probably be a while before I even have a simple guest running :/
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