From: Paolo Bonzini <pbonzini@redhat.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers
Date: Wed, 16 Oct 2019 19:01:25 +0200 [thread overview]
Message-ID: <0e290a9d-9d26-d24a-ba01-9fda4826a5ac@redhat.com> (raw)
In-Reply-To: <20191016165057.GJ6487@redhat.com>
On 16/10/19 18:50, Andrea Arcangeli wrote:
>> It still doesn't add up. 0.3ms / 5 is 1/15000th of a second; 43us is
>> 1/25000th of a second. Do you have multiple vCPU perhaps?
>
> Why would I run any test on UP guests? Rather then spending time doing
> the math on my results, it's probably quicker that you run it yourself:
I don't know, but if you don't say how many vCPUs you have, I cannot do
the math and review the patch.
>> The number of vmexits doesn't count (for HLT). What counts is how long
>> they take to be serviced, and as long as it's 1us or more the
>> optimization is pointless.
>
> Please note the single_task_running() check which immediately breaks
> the kvm_vcpu_check_block() loop if there's even a single other task
> that can be scheduled in the runqueue of the host CPU.
>
> What happen when the host is not idle is quoted below:
>
> w/o optimization with optimization
> ---------------------- -------------------------
> 0us vmexit vmexit
> 500ns retpoline call vmexit handler directly
> 600ns retpoline kvm_vcpu_check_block()
> 700ns retpoline schedule()
> 800ns kvm_vcpu_check_block()
> 900ns schedule()
> ...
>
> Disclaimer: the numbers on the left are arbitrary and I just cut and
> pasted them from yours, no idea how far off they are.
Yes, of course. But the idea is the same: yes, because of the retpoline
you run the guest for perhaps 300ns more before schedule()ing, but does
that really matter? 300ns * 20000 times/second is a 0.6% performance
impact, and 300ns is already very generous. I am not sure it would be
measurable at all.
Paolo
> To be clear, I would find it very reasonable to be requested to proof
> the benefit of the HLT optimization with benchmarks specifics for that
> single one liner, but until then, the idea that we can drop the
> retpoline optimization from the HLT vmexit by just thinking about it,
> still doesn't make sense to me, because by thinking about it I come to
> the opposite conclusion.
>
> The lack of single_task_running() in the guest driver is also why the
> guest cpuidle haltpoll risks to waste some CPU with host overcommit or
> with the host loaded at full capacity and why we may not assume it to
> be universally enabled.
>
> Thanks,
> Andrea
>
next prev parent reply other threads:[~2019-10-16 17:01 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-28 17:23 [PATCH 00/14] KVM monolithic v2 Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 01/14] KVM: monolithic: x86: remove kvm.ko Andrea Arcangeli
2019-10-15 1:31 ` Sean Christopherson
2019-10-15 3:18 ` Sean Christopherson
2019-10-15 8:32 ` Paolo Bonzini
2019-09-28 17:23 ` [PATCH 02/14] KVM: monolithic: x86: disable linking vmx and svm at the same time into the kernel Andrea Arcangeli
2019-10-15 3:16 ` Sean Christopherson
2019-10-15 8:21 ` Paolo Bonzini
2019-10-15 15:23 ` Sean Christopherson
2019-09-28 17:23 ` [PATCH 04/14] KVM: monolithic: x86: handle the request_immediate_exit variation Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 05/14] KVM: monolithic: add more section prefixes in the KVM common code Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 06/14] KVM: monolithic: x86: remove __exit section prefix from machine_unsetup Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 07/14] KVM: monolithic: x86: remove __init section prefix from kvm_x86_cpu_has_kvm_support Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 08/14] KVM: monolithic: x86: remove exports Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 09/14] KVM: monolithic: remove exports from KVM common code Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 10/14] KVM: monolithic: x86: drop the kvm_pmu_ops structure Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 11/14] KVM: x86: optimize more exit handlers in vmx.c Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers Andrea Arcangeli
2019-10-15 8:28 ` Paolo Bonzini
2019-10-15 16:49 ` Andrea Arcangeli
2019-10-15 19:46 ` Paolo Bonzini
2019-10-15 20:35 ` Andrea Arcangeli
2019-10-15 22:22 ` Paolo Bonzini
2019-10-15 23:42 ` Andrea Arcangeli
2019-10-16 7:07 ` Paolo Bonzini
2019-10-16 16:50 ` Andrea Arcangeli
2019-10-16 17:01 ` Paolo Bonzini [this message]
2019-09-28 17:23 ` [PATCH 13/14] KVM: retpolines: x86: eliminate retpoline from svm.c " Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 14/14] x86: retpolines: eliminate retpoline from msr event handlers Andrea Arcangeli
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