linux-kernel - Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

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Message-ID: <4F74A405.2040609@linux.vnet.ibm.com>
Date:	Thu, 29 Mar 2012 23:33:49 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Alan Meadows <alan.meadows@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Attilio Rao <attilio.rao@...rix.com>
Subject: Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

On 03/29/2012 03:28 PM, Avi Kivity wrote:
> On 03/28/2012 08:21 PM, Raghavendra K T wrote:
>>
>>>
>>>
>>>          Looks like a good baseline on which to build the KVM
>>>          implementation.  We
>>>          might need some handshake to prevent interference on the host
>>>          side with
>>>          the PLE code.
>>>
>>
>> I think I still missed some point in Avi's comment. I agree that PLE
>> may be interfering with above patches (resulting in less performance
>> advantages). but we have not seen performance degradation with the
>> patches in earlier benchmarks. [ theoretically since patch has very
>> slight advantage over PLE that atleast it knows who should run next ].
>
> The advantage grows with the vcpu counts and overcommit ratio.  If you
> have N vcpus and M:1 overcommit, PLE has to guess from N/M queued vcpus
> while your patch knows who to wake up.
>

Yes. I agree.

>>
>> So TODO in my list on this is:
>> 1. More analysis of performance on PLE mc.
>> 2. Seeing how to implement handshake to increase performance (if PLE +
>> patch combination have slight negative effect).
>
> I can think of two options:

I really like below ideas. Thanks for that!.

> - from the PLE handler, don't wake up a vcpu that is sleeping because it
> is waiting for a kick

How about, adding another pass in the beginning of  kvm_vcpu_on_spin()
to check if any vcpu is already kicked. This would almost result in 
yield_to(kicked_vcpu). IMO this is also worth trying.

will try above ideas soon.

> - look at other sources of pause loops (I guess smp_call_function() is
> the significant source) and adjust them to use the same mechanism, and
> ask the host to disable PLE exiting.
>
> This can be done incrementally later.
>

Yes.. this can wait a bit.

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