linux-kernel - Re: [RFC PATCH 3/3] kvm: use yield_to instead of sleep in kvm_vcpu_on

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Message-ID: <4D0008D5.1040102@redhat.com>
Date:	Wed, 08 Dec 2010 17:38:13 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 3/3] kvm: use yield_to instead of sleep in kvm_vcpu_on_spin

On 12/05/2010 07:56 AM, Avi Kivity wrote:

>> + if (vcpu == me)
>> + continue;
>> + if (vcpu->spinning)
>> + continue;
>
> You may well want to wake up a spinner. Suppose
>
> A takes a lock
> B preempts A
> B grabs a ticket, starts spinning, yields to A
> A releases lock
> A grabs ticket, starts spinning
>
> at this point, we want A to yield to B, but it won't because of this check.

That's a good point.  I guess we'll have to benchmark both with
and without the vcpu->spinning logic.

>> + if (!task)
>> + continue;
>> + if (waitqueue_active(&vcpu->wq))
>> + continue;
>> + if (task->flags& PF_VCPU)
>> + continue;
>> + kvm->last_boosted_vcpu = i;
>> + yield_to(task);
>> + break;
>> + }
>
> I think a random selection algorithm will be a better fit against
> special guest behaviour.

Possibly, though I suspect we'd have to hit very heavy overcommit ratios
with very large VMs before round robin stops working.

>> - /* Sleep for 100 us, and hope lock-holder got scheduled */
>> - expires = ktime_add_ns(ktime_get(), 100000UL);
>> - schedule_hrtimeout(&expires, HRTIMER_MODE_ABS);
>> + if (first_round&& last_boosted_vcpu == kvm->last_boosted_vcpu) {
>> + /* We have not found anyone yet. */
>> + first_round = 0;
>> + goto again;
>
> Need to guarantee termination.

We do that by setting first_round to 0 :)

We at most walk N+1 VCPUs in a VM with N VCPUs, with
this patch.

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