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

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Date:	Sun, 01 Apr 2012 19:18:57 +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 04/01/2012 06:48 PM, Avi Kivity wrote:
> On 03/30/2012 01:07 PM, Raghavendra K T wrote:
>> On 03/29/2012 11:33 PM, Raghavendra K T wrote:
>>> On 03/29/2012 03:28 PM, Avi Kivity wrote:
>>>> On 03/28/2012 08:21 PM, Raghavendra K T wrote:
>>
>>> 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.
>>>
>>
>> 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?

  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.

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