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

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Message-ID: <4F785DCF.7020809@redhat.com>
Date:	Sun, 01 Apr 2012 16:53:19 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.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 04/01/2012 04:48 PM, Raghavendra K T wrote:
>>> I have patch something like below in mind to try:
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index d3b98b1..5127668 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -1608,15 +1608,18 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>>>        * else and called schedule in __vcpu_run.  Hopefully that
>>>        * VCPU is holding the lock that we need and will release it.
>>>        * We approximate round-robin by starting at the last boosted
>>> VCPU.
>>> +     * Priority is given to vcpu that are unhalted.
>>>        */
>>> -    for (pass = 0; pass<  2&&  !yielded; pass++) {
>>> +    for (pass = 0; pass<  3&&  !yielded; pass++) {
>>>           kvm_for_each_vcpu(i, vcpu, kvm) {
>>>               struct task_struct *task = NULL;
>>>               struct pid *pid;
>>> -            if (!pass&&  i<  last_boosted_vcpu) {
>>> +            if (!pass&&  !vcpu->pv_unhalted)
>>> +                continue;
>>> +            else if (pass == 1&&  i<  last_boosted_vcpu) {
>>>                   i = last_boosted_vcpu;
>>>                   continue;
>>> -            } else if (pass&&  i>  last_boosted_vcpu)
>>> +            } else if (pass == 2&&  i>  last_boosted_vcpu)
>>>                   break;
>>>               if (vcpu == me)
>>>                   continue;
>>>
>>
>> Actually I think this is unneeded.  The loops tries to find vcpus that
>> are runnable but not running (vcpu_active(vcpu->wq)), and halted vcpus
>> don't match this condition.
>>
>
>
> I almost agree. But at corner of my thought,
>
> Suppose there are 8 vcpus runnable out of which 4 of them are kicked
> but not running, making yield_to those 4 vcpus would result in better
> lock progress. no?

That's what the code does.

>   I still have little problem getting PLE setup, here (instead
> rebasing patches).
> Once I get PLE to get that running, and numbers prove no improvement,
> I will drop this idea.
>

I'm interested in how PLE does vs. your patches, both with PLE enabled
and disabled.

-- 
error compiling committee.c: too many arguments to function

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