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

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Date:   Fri, 23 Jun 2017 14:49:04 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     Wanpeng Li <kernellwp@...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

On 2017/6/23 12:35, Wanpeng Li wrote:
> 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.

This mechanism is not specific to KVM. It is a kernel feature which can 
benefit guest when running inside X86 virtualization environment. The 
guest includes KVM,Xen,VMWARE,Hyper-v. Administrator can control KVM to 
use adaptive halt poll but he cannot control the user to use halt 
polling inside guest. Lots of user set idle=poll inside guest to improve 
performance which occupy more CPU cycles. This mechanism is a 
enhancement to it not to KVM halt polling.

>
> Regards,
> Wanpeng Li
>


-- 
Yang
Alibaba Cloud Computing