linux-kernel - Re: [PATCH] KVM: add halt_attempted

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Date:	Wed, 16 Sep 2015 20:12:24 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
CC:	rkrcmar@...hat.com, David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	David Matlack <dmatlack@...gle.com>,
	Jens Freimann <jfrei@...ux.vnet.ibm.com>
Subject: Re: [PATCH] KVM: add halt_attempted_poll to VCPU stats

On 9/16/15 6:12 PM, Christian Borntraeger wrote:
> Am 15.09.2015 um 18:27 schrieb Paolo Bonzini:
>> This new statistic can help diagnosing VCPUs that, for any reason,
>> trigger bad behavior of halt_poll_ns autotuning.
>>
>> For example, say halt_poll_ns = 480000, and wakeups are spaced exactly
>> like 479us, 481us, 479us, 481us. Then KVM always fails polling and wastes
>> 10+20+40+80+160+320+480 = 1110 microseconds out of every
>> 479+481+479+481+479+481+479 = 3359 microseconds. The VCPU then

For the first 481 us, block_ns should be 481us, block_ns > 
halt_poll_ns(480us) and long halt is detected, the vcpu->halt_poll_ns 
will be shrinked.

>> is consuming about 30% more CPU than it would use without
>> polling.  This would show as an abnormally high number of
>> attempted polling compared to the successful polls.
>>
>> Cc: Christian Borntraeger <borntraeger@...ibm.com<
>> Cc: David Matlack <dmatlack@...gle.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Acked-by: Christian Borntraeger <borntraeger@...ibm.com>
>
> yes, this will help to detect some bad cases, but not all.
>
> PS:
> upstream maintenance keeps me really busy at the moment :-)
> I am looking into a case right now, where auto polling goes
> completely nuts on my system:
>
> guest1: 8vcpus		guest2: 1 vcpu
> iperf with 25 process (-P25) from guest1 to guest2.
>
> I/O interrupts on s390 are floating (pending on all CPUs) so on
> ALL VCPUs that go to sleep, polling will consider any pending
> network interrupt as successful poll. So with auto polling the
> guest consumes up to 5 host CPUs without auto polling only 1.
> Reducing  halt_poll_ns to 100000 seems to work (goes back to
> 1 cpu).
>
> The proper way might be to feedback the result of the
> interrupt dequeue into the heuristics. Don't know yet how
> to handle that properly.

If this can be reproduced on x86 platform?

Regards,
Wanpeng Li
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