linux-kernel - Re: [PATCH 1/2] x86/paravirt: Don't make vcpu_is

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Message-ID: <2bad716f-8e78-d9c0-56c9-61f5d52ab97a@redhat.com>
Date:   Wed, 8 Feb 2017 15:17:45 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jeremy Fitzhardinge <jeremy@...p.org>,
        Chris Wright <chrisw@...s-sol.org>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
        Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 1/2] x86/paravirt: Don't make vcpu_is_preempted() a
 callee-save function

On 02/08/2017 02:05 PM, Peter Zijlstra wrote:
> On Wed, Feb 08, 2017 at 01:00:24PM -0500, Waiman Long wrote:
>> It was found when running fio sequential write test with a XFS ramdisk
>> on a 2-socket x86-64 system, the %CPU times as reported by perf were
>> as follows:
>>
>>  71.27%  0.28%  fio  [k] down_write
>>  70.99%  0.01%  fio  [k] call_rwsem_down_write_failed
>>  69.43%  1.18%  fio  [k] rwsem_down_write_failed
>>  65.51% 54.57%  fio  [k] osq_lock
>>   9.72%  7.99%  fio  [k] __raw_callee_save___kvm_vcpu_is_preempted
>>   4.16%  4.16%  fio  [k] __kvm_vcpu_is_preempted
>>
>> So making vcpu_is_preempted() a callee-save function has a pretty high
>> cost associated with it. As vcpu_is_preempted() is called within the
>> spinlock, mutex and rwsem slowpaths, there isn't much to gain by making
>> it callee-save. So it is now changed to a normal function call instead.
>>
> Numbers for bare metal too please.

I will run the test on bare metal, but I doubt there will be noticeable
difference.

Cheers,
Longman