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

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Message-ID: <df6b82dd-1219-c2d1-8f14-bd4735bf4bd3@suse.de>
Date:   Fri, 14 Jul 2017 11:37:20 +0200
From:   Alexander Graf <agraf@...e.de>
To:     Yang Zhang <yang.zhang.wz@...il.com>,
        Radim Krčmář <rkrcmar@...hat.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



On 13.07.17 13:49, Yang Zhang wrote:
> On 2017/7/4 22:13, Radim Krčmář wrote:
>> 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?
> 
> One is to remove the timer and another one is to reprogram the timer. 
> Normally, only one write to remove the timer.But in some cases, it will 
> reprogram it.
> 
>>
>>> 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?
> 
> In the worst case, each timer will touch APIC timer twice.So it will add 
> additional 4 msr writse. But this is  not always true.
> 
>>
>>>                                                          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.
> 
> Yes, i can try it. But MWAIT will not yield CPU, it only helps the 
> sibling hyperthread as you mentioned.

If you implement proper MWAIT emulation that conditionally gets en- or 
disabled depending on the same halt poll dynamics that we already have 
for in-host HLT handling, it will also yield the CPU.

As for the timer - are you sure the problem is really the overhead of 
the timer configuration, not the latency that it takes to actually fire 
the guest timer?

One major problem I see is that we configure the host hrtimer to fire at 
the point in time when the guest wants to see a timer event. But in a 
virtual environment, the point in time when we have to start switching 
to the VM really should be a bit *before* the guest wants to be woken 
up, as it takes quite some time to switch back into the VM context.


Alex