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

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Message-ID: <20170704141322.GC30880@potion>
Date:   Tue, 4 Jul 2017 16:13:23 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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

2017-07-03 17:28+0800, Yang Zhang:
> 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

One write is enough to disable/re-enable the APIC timer -- why does
Linux use two?

> 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.

Yeah, sounds like too much ... I understood that there are

  IPI from 1 to 2
  4 * APIC timer
  IPI from 2 to 1

which adds to 6 MSR writes -- what are the other 4?

>                                                          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. Same for halt-poll in KVM, it only solve the
> cost from schedule in/out in host and can not help such workload much.
> 
> The purpose of this patch we want to improve current idle=poll mechanism to

Please aim to allow MWAIT instead of idle=poll -- MWAIT doesn't slow
down the sibling hyperthread.  MWAIT solves the IPI problem, but doesn't
get rid of the timer one.

> use dynamic polling and do poll before touch sched timer. It should not be a
> virtualization specific feature but seems bare mental have low cost to
> access the MSR. So i want to only enable it in VM. Though the idea below the
> patch may not so perfect to fit all conditions, it looks no worse than now.

It adds code to hot-paths (interrupt handlers) while trying to optimize
an idle-path, which is suspicious.

> How about we keep current implementation and i integrate the patch to
> para-virtualize part as Paolo suggested? We can continue discuss it and i
> will continue to refine it if anyone has better suggestions?

I think there is a nicer solution to avoid the expensive timer rewrite:
Linux uses one-shot APIC timers and getting the timer interrupt is about
as expensive as programming the timer, so the guest can keep the timer
armed, but not re-arm it after the expiration if the CPU is idle.

This should also mitigate the problem with short idle periods, but the
optimized window is anywhere between 0 to 1ms.

Do you see disadvantages of this combined with MWAIT?

Thanks.