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

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Date:	Fri, 27 Mar 2015 12:10:48 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Waiman.Long@...com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, paolo.bonzini@...il.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
	riel@...hat.com, torvalds@...ux-foundation.org,
	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 03/16/2015 06:46 PM, Peter Zijlstra wrote:
> Hi Waiman,
>
> As promised; here is the paravirt stuff I did during the trip to BOS last week.
>
> All the !paravirt patches are more or less the same as before (the only real
> change is the copyright lines in the first patch).
>
> The paravirt stuff is 'simple' and KVM only -- the Xen code was a little more
> convoluted and I've no real way to test that but it should be stright fwd to
> make work.
>
> I ran this using the virtme tool (thanks Andy) on my laptop with a 4x
> overcommit on vcpus (16 vcpus as compared to the 4 my laptop actually has) and
> it both booted and survived a hackbench run (perf bench sched messaging -g 20
> -l 5000).
>
> So while the paravirt code isn't the most optimal code ever conceived it does work.
>
> Also, the paravirt patching includes replacing the call with "movb $0, %arg1"
> for the native case, which should greatly reduce the cost of having
> CONFIG_PARAVIRT_SPINLOCKS enabled on actual hardware.
>
> I feel that if someone were to do a Xen patch we can go ahead and merge this
> stuff (finally!).
>
> These patches do not implement the paravirt spinlock debug stats currently
> implemented (separately) by KVM and Xen, but that should not be too hard to do
> on top and in the 'generic' code -- no reason to duplicate all that.
>
> Of course; once this lands people can look at improving the paravirt nonsense.
>

last time I had reported some hangs in kvm case, and I can confirm that
the current set of patches works fine.

Feel free to add
Tested-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> #kvm pv

As far as performance is concerned (with my 16core +ht machine having
16vcpu guests [ even w/ , w/o the lfsr hash patchset ]), I do not see
any significant observations to report, though I understand that we
could see much more benefit with large number of vcpus because of
possible reduction in cache bouncing.






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