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

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Date:   Tue, 27 Jun 2017 16:22:51 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Yang Zhang <yang.zhang.wz@...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,
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        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

2017-06-27 15:56+0200, Paolo Bonzini:
> On 27/06/2017 15:40, Radim Krčmář wrote:
>>> ... which is not necessarily _wrong_.  It's just a different heuristic.
>> Right, it's just harder to use than host's single_task_running() -- the
>> VCPU calling vcpu_is_preempted() is never preempted, so we have to look
>> at other VCPUs that are not halted, but still preempted.
>> 
>> If we see some ratio of preempted VCPUs (> 0?), then we stop polling and
>> yield to the host.  Working under the assumption that there is work for
>> this PCPU if other VCPUs have stuff to do.  The downside is that it
>> misses information about host's topology, so it would be hard to make it
>> work well.
> 
> I would just use vcpu_is_preempted on the current CPU.  From guest POV
> this option is really a "f*** everyone else" setting just like
> idle=poll, only a little more polite.

vcpu_is_preempted() on current cpu cannot return true, AFAIK.

> If we've been preempted and we were polling, there are two cases.  If an
> interrupt was queued while the guest was preempted, the poll will be
> treated as successful anyway.

I think the poll should be treated as invalid if the window has expired
while the VCPU was preempted -- the guest can't tell whether the
interrupt arrived still within the poll window (unless we added paravirt
for that), so it shouldn't be wasting time waiting for it.

>                                If it hasn't, let others run---but really
> that's not because the guest wants to be polite, it's to avoid that the
> scheduler penalizes it excessively.

This sounds like a VM entry just to do an immediate VM exit, so paravirt
seems better here as well ... (the guest telling the host about its
window -- which could also be used to rule it out as a target in the
pause loop random kick.)

> So until it's preempted, I think it's okay if the guest doesn't care
> about others.  You wouldn't use this option anyway in overcommitted
> situations.
> 
> (I'm still not very convinced about the idea).

Me neither.  (The same mechanism is applicable to bare-metal, but was
never used there, so I would rather bring the guest behavior closer to
bare-metal.)