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

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Message-ID: <20170627140008.GA1503@potion>
Date:   Tue, 27 Jun 2017 16:00:08 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        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,
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        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 14:49+0800, Yang Zhang:
> 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.

Users of idle=poll shouln't overcommit, so the goal seems to be energy
savings without crippling the guest performance too much ...

Wouldn't switching to idle=mwait work as well?

Thanks.