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

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Message-ID: <20150905174519.GU3644@twins.programming.kicks-ass.net>
Date:	Sat, 5 Sep 2015 19:45:19 +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 Fri, Sep 04, 2015 at 08:58:38AM -0700, Linus Torvalds wrote:
> On Fri, Sep 4, 2015 at 8:30 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >> And even ignoring the "implementation was crap" issue, some people may
> >> well want their kernels to be "bare hardware" kernels even under a
> >> hypervisor. It may be a slim hypervisor that gives you all the cpus,
> >> or it may just be a system that is just sufficiently overprovisioned,
> >> so you don't get vcpu preemption in practice.
> >
> > Fair enough; I had not considered the slim hypervisor case.
> >
> > Should I place the virt_spin_lock() thing under CONFIG_PARAVIRT (maybe
> > even _SPINLOCKS) such that only paravirt enabled kernels when ran on a
> > hypervisor that does not support paravirt patching (HyperV, VMware,
> > etc..) revert to the test-and-set?
> 
> My gut feel would be to try to match out old paravirt setup, which
> similarly replaced the ticket locks with the test-and-set lock, and
> try to match the situation where that happened?

I'm not sure there was a test-and-set option in 4.1.

Either the hypervisor layer implemented paravirt spinlocks (Xen, KVM)
(and you selected CONFIG_PARAVIRT_SPINLOCKS, which had a fairly large
negative impact on native code), or you got our native locking.

So if you want I can simply remove the whole test-and-set thing, but I'd
rather fix it and put it under one of the PARAVIRT options.

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