linux-kernel - Re: [PATCH v3] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Date:	Tue, 24 May 2016 15:05:54 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	David Matlack <dmatlack@...gle.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v3] KVM: halt-polling: poll if emulated lapic timer will
 fire soon

2016-05-24 10:19 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2016-05-24 2:01 GMT+08:00 David Matlack <dmatlack@...gle.com>:
>> On Sun, May 22, 2016 at 5:42 PM, Wanpeng Li <kernellwp@...il.com> wrote:
>>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> I'm ok with this patch, but I'd like to better understand the target
>> workloads. What type of workloads do you expect to benefit from this?
>
> dynticks guests I think is one of workloads which can get benefit,
> there are lots of upcoming fire timers captured by my feature. Even
> during TCP testing. And also the workload of Yang's.
>
>>
>>>
>>> If an emulated lapic timer will fire soon(in the scope of 10us the
>>> base of dynamic halt-polling, lower-end of message passing workload
>>> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
>>> and poll to wait it fire, the fire callback apic_timer_fn() will set
>>> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
>>> This can avoid context switch overhead and the latency which we wake
>>> up vCPU.
>>>
>>> This feature is slightly different from current advance expiration
>>> way. Advance expiration rely on the vCPU is running(do polling before
>>> vmentry). But in some cases, the timer interrupt may be blocked by
>>> other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to
>>> run immediately. So even advance the timer early, vCPU may still see
>>> the latency. But polling is different, it ensures the vCPU to aware
>>> the timer expiration before schedule out.
>>>
>>> iperf TCP get ~6% bandwidth improvement.
>>
>> I think my question got lost in the previous thread :). Can you
>> explain why TCP bandwidth improves with this patch?
>

Please forget TCP stuff. I run lmbench ctx switch benchmark:

echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests.

Context switching - times in microseconds - smaller is better
-------------------------------------------------------------------------
Host                 OS  2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
                         ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
kernel     Linux 4.6.0+ 7.9800   11.0   10.8   14.6 9.4300    13.0
10.2 vanilla
kernel     Linux 4.6.0+   15.3   13.6   10.7   12.5 9.0000    12.8 7.38000 poll

Regards,
Wanpeng Li