linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Message-ID: <4D511327.1020102@siemens.com>
Date:	Tue, 08 Feb 2011 10:55:51 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Zachary Amsden <zamsden@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

On 2011-02-08 10:15, Avi Kivity wrote:
> On 02/07/2011 07:23 PM, Jan Kiszka wrote:
>> On 2011-02-07 18:10, Avi Kivity wrote:
>>>  On 02/07/2011 06:59 PM, Jan Kiszka wrote:
>>>>>
>>>>>   (well, actually, cpufreq_notifier and kvm_arch_hardware_enable are
>>>>>   already non preemptible, and the stats code should just go away?)
>>>>
>>>>  The stats code is trivial to convert, so it doesn't matter.
>>>
>>>  Removal is easier.
>>
>> Is that stat interface no longer used?
> 
> It's there for compatibility.  I'm itching to remove it.  See 
> qemu-kvm.git/kvm/kvm_stat for the only known user, and for the 
> replacement via tracepoints.

OK, but that will first take a grace period.

> 
> Tracepoints have marginally lower overhead when disabled, and somewhat 
> higher overhead when enabled.  A disadvantage of tracepoints is that it 
> is harder to associate an event with a vm when that event is triggered 
> by a workqueue, but I don't think it matters in practice (kvm_stat 
> doesn't even provide a per-vm breakdown).

What about using the perf infrastructure for this? Besides that perf can
reuse tracepoints, maybe there is even a more efficient way of added new
stat sources.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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