linux-kernel - Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

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Date:	Tue, 11 Dec 2012 11:37:59 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	John Stultz <john.stultz@...aro.org>
Cc:	stefani@...bold.net, linux-kernel@...r.kernel.org,
	ak@...ux.intel.com, aarcange@...hat.com
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

On Tue, Dec 11, 2012 at 11:27 AM, John Stultz <john.stultz@...aro.org> wrote:
> On 12/11/2012 08:11 AM, stefani@...bold.net wrote:
>>
>> From: Stefani Seibold <stefani@...bold.net>
>>
>> This small patch add the functions vdso_gettimeofday(),
>> vdso_clock_gettime()
>> and vdso_time() support to the VDSO for x86 32-bit kernels.
>>
>> The reason to do this was to get a fast reliable time stamp. Many
>> developers
>> uses TSC to get a fast time time stamp, without knowing the pitfalls. VDSO
>> time functions a fast and reliable way, because the kernel knows the best
>> time
>> source and the P- and C-state of the CPU.
>
> Very cool. There have been similar implementations of this patch over the
> years, but they were all bit more hackish then this.
>
>
>
>> For x86 the vclock_gettime.c currently supports only the HPET and TSC
>> timer,
>> the ACPI timer should be easily to add with an other patch.
>
> Although the ACPI PM timer requires port-io which would need tweaking to
> allow normal users to access it. And I'm not sure if the performance would
> be much improved, as the port-io probably dominates the performance cost.
>

I'd actually advocate going the other way and removing hpet vdso
support.  Last time I benchmarked this, a syscall took about 20ns on
my box, and reading the hpet counter took about 750ns, giving
approximately no gain to the hpet-via-vdso optimization.

--Andy
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