Re: [PATCH 0/2] x86/idle: add halt poll support

From: Wanpeng Li
Date: Fri Jun 23 2017 - 00:36:23 EST

2017-06-23 12:08 GMT+08:00 Yang Zhang <yang.zhang.wz@xxxxxxxxx>:
> On 2017/6/22 19:50, Wanpeng Li wrote:
>> 2017-06-22 19:22 GMT+08:00 root <yang.zhang.wz@xxxxxxxxx>:
>>> From: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
>>> Some latency-intensive workload will see obviously performance
>>> drop when running inside VM. The main reason is that the overhead
>>> is amplified when running inside VM. The most cost i have seen is
>>> inside idle path.
>>> This patch introduces a new mechanism to poll for a while before
>>> entering idle state. If schedule is needed during poll, then we
>>> don't need to goes through the heavy overhead path.
>>> Here is the data i get when running benchmark contextswitch
>>> (
>>> before patch:
>>> 2000000 process context switches in 4822613801ns (2411.3ns/ctxsw)
>>> after patch:
>>> 2000000 process context switches in 3584098241ns (1792.0ns/ctxsw)
>> If you test this after disabling the adaptive halt-polling in kvm?
>> What's the performance data of w/ this patchset and w/o the adaptive
>> halt-polling in kvm, and w/o this patchset and w/ the adaptive
>> halt-polling in kvm? In addition, both linux and windows guests can
>> get benefit as we have already done this in kvm.
> I will provide more data in next version. But it doesn't conflict with

Another case I can think of is w/ both this patchset and the adaptive
halt-polling in kvm.

> current halt polling inside kvm. This is just another enhancement.

I didn't look close to the patchset, however, maybe there is another
poll in the kvm part again sometimes if you fails the poll in the
guest. In addition, the adaptive halt-polling in kvm has performance
penalty when the pCPU is heavily overcommitted though there is a
single_task_running() in my testing, it is hard to accurately aware
whether there are other tasks waiting on the pCPU in the guest which
will make it worser. Depending on vcpu_is_preempted() or steal time
maybe not accurately or directly.

So I'm not sure how much sense it makes by adaptive halt-polling in
both guest and kvm. I prefer to just keep adaptive halt-polling in
kvm(then both linux/windows or other guests can get benefit) and avoid
to churn the core x86 path.

Wanpeng Li