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

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Date:   Fri, 23 Jun 2017 12:35:59 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Jonathan Corbet <corbet@....net>, tony.luck@...el.com,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, mchehab@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>, krzk@...nel.org,
        jpoimboe@...hat.com, Andy Lutomirski <luto@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Robert Gerst <rgerst@...il.com>,
        Mathias Krause <minipli@...glemail.com>,
        douly.fnst@...fujitsu.com, Nicolai Stange <nicstange@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>, dvlasenk@...hat.com,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        yamada.masahiro@...ionext.com, mika.westerberg@...ux.intel.com,
        Chen Yu <yu.c.chen@...el.com>, aaron.lu@...el.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Kyle Huey <me@...ehuey.com>, Len Brown <len.brown@...el.com>,
        Prarit Bhargava <prarit@...hat.com>,
        hidehiro.kawai.ez@...achi.com, fengtiantian@...wei.com,
        pmladek@...e.com, jeyu@...hat.com, Larry.Finger@...inger.net,
        zijun_hu@....com, luisbg@....samsung.com, johannes.berg@...el.com,
        niklas.soderlund+renesas@...natech.se, zlpnobody@...il.com,
        Alexey Dobriyan <adobriyan@...il.com>, fgao@...ai8.com,
        ebiederm@...ssion.com,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Arnd Bergmann <arnd@...db.de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, linux-edac@...r.kernel.org,
        kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 0/2] x86/idle: add halt poll support

2017-06-23 12:08 GMT+08:00 Yang Zhang <yang.zhang.wz@...il.com>:
> On 2017/6/22 19:50, Wanpeng Li wrote:
>>
>> 2017-06-22 19:22 GMT+08:00 root <yang.zhang.wz@...il.com>:
>>>
>>> From: Yang Zhang <yang.zhang.wz@...il.com>
>>>
>>> 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
>>> (https://github.com/tsuna/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.

Regards,
Wanpeng Li