From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>,
Yang Zhang <yang.zhang.wz@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
the arch/x86 maintainers <x86@kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
tony.luck@intel.com, Borislav Petkov <bp@alien8.de>,
Peter Zijlstra <peterz@infradead.org>,
mchehab@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
krzk@kernel.org, jpoimboe@redhat.com,
Andy Lutomirski <luto@kernel.org>,
Christian Borntraeger <borntraeger@de.ibm.com>,
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Robert Gerst <rgerst@gmail.com>,
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Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
Arnd Bergmann <arnd@arndb.de>,
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kvm <kvm@vger.kernel.org>
Subject: Re: [PATCH 2/2] x86/idle: use dynamic halt poll
Date: Tue, 27 Jun 2017 16:22:51 +0200 [thread overview]
Message-ID: <20170627142251.GB1487@potion> (raw)
In-Reply-To: <2771f905-d1b0-b118-9ae9-db5fb87f877c@redhat.com>
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.)
next prev parent reply other threads:[~2017-06-27 14:23 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-22 11:22 [PATCH 0/2] x86/idle: add halt poll support root
2017-06-22 11:22 ` [PATCH 1/2] x86/idle: add halt poll for halt idle root
2017-06-22 14:23 ` Thomas Gleixner
2017-06-23 4:05 ` Yang Zhang
2017-08-16 4:04 ` Michael S. Tsirkin
2017-08-17 7:29 ` Yang Zhang
2017-06-22 11:22 ` [PATCH 2/2] x86/idle: use dynamic halt poll root
2017-06-22 11:51 ` Paolo Bonzini
2017-06-23 3:58 ` Yang Zhang
2017-06-27 11:22 ` Yang Zhang
2017-06-27 12:07 ` Paolo Bonzini
2017-06-27 12:23 ` Wanpeng Li
2017-06-27 12:28 ` Paolo Bonzini
2017-06-27 13:40 ` Radim Krčmář
2017-06-27 13:56 ` Paolo Bonzini
2017-06-27 14:22 ` Radim Krčmář [this message]
2017-06-27 14:26 ` Paolo Bonzini
2017-07-03 9:28 ` Yang Zhang
2017-07-03 10:06 ` Thomas Gleixner
2017-07-04 2:19 ` Yang Zhang
2017-07-04 14:13 ` Radim Krčmář
2017-07-04 14:50 ` Thomas Gleixner
2017-07-13 11:49 ` Yang Zhang
2017-07-14 9:37 ` Alexander Graf
2017-07-17 9:26 ` Yang Zhang
2017-07-17 9:54 ` Alexander Graf
2017-07-17 12:50 ` Yang Zhang
2017-07-04 22:28 ` Wanpeng Li
2017-06-22 14:32 ` Thomas Gleixner
2017-06-23 4:04 ` Yang Zhang
2017-06-22 22:46 ` kbuild test robot
2017-06-22 11:32 ` [PATCH 0/2] x86/idle: add halt poll support Yang Zhang
2017-06-22 11:50 ` Wanpeng Li
2017-06-23 4:08 ` Yang Zhang
2017-06-23 4:35 ` Wanpeng Li
2017-06-23 6:49 ` Yang Zhang
2017-06-27 14:00 ` Radim Krčmář
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