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

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Date:	Fri, 4 Sep 2015 08:21:28 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.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 4, 2015 at 8:14 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> The reason we chose to revert to a test-and-set is because regular fair
> locks, like the ticket and the queue thing, have horrible behaviour
> under vcpu preemption.

Right. However, with our old ticket locks, that's what we got when you
didn't ask for paravirt support. No?

So this seems to be a misfeature - you made the hypervisor "support"
unconditional.  Even a kernel compiled for raw hardware now does that
"let's act differently under a hypervisor", which I think is quite
debatable to begin with, but when that "act differently" is then
complete garbage, it's a disaster.

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.

But it would be interesting to hear if just fixing the busy-looping to
not pound the lock with a constant stream of cmpxchg's is already
sufficient to fix the big picture problem.

                    Linus
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