linux-kernel - Re: [PATCH 2/2] x86/idle: use dynamic halt poll

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Date:   Mon, 3 Jul 2017 12:06:19 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yang Zhang <yang.zhang.wz@...il.com>
cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.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 2/2] x86/idle: use dynamic halt poll

On Mon, 3 Jul 2017, Yang Zhang wrote:
> The background is that we(Alibaba Cloud) do get more and more complaints from
> our customers in both KVM and Xen compare to bare-mental.After investigations,
> the root cause is known to us: big cost in message passing workload(David show
> it in KVM forum 2015)
> 
> A typical message workload like below:
> vcpu 0                             vcpu 1
> 1. send ipi                     2.  doing hlt
> 3. go into idle                 4.  receive ipi and wake up from hlt
> 5. write APIC time twice        6.  write APIC time twice to
>    to stop sched timer              reprogram sched timer
> 7. doing hlt                    8.  handle task and send ipi to
>                                     vcpu 0
> 9. same to 4.                   10. same to 3
> 
> One transaction will introduce about 12 vmexits(2 hlt and 10 msr write). The
> cost of such vmexits will degrades performance severely. Linux kernel already
> provide idle=poll to mitigate the trend. But it only eliminates the IPI and
> hlt vmexit. It has nothing to do with start/stop sched timer. A compromise
> would be to turn off NOHZ kernel, but it is not the default config for new
> distributions.

You still can turn if off on the kernel command line via nohz=off

Thanks,

	tglx