linux-kernel - Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO

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Message-ID: <54A6D0B6.7090700@gmail.com>
Date:	Fri, 02 Jan 2015 10:09:10 -0700
From:	David Ahern <dsahern@...il.com>
To:	Shaohua Li <shli@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Kernel-team@...com,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO

On 1/2/15 10:02 AM, Shaohua Li wrote:
> On Fri, Jan 02, 2015 at 08:31:33AM -0700, David Ahern wrote:
>> On 1/1/15 7:59 PM, Shaohua Li wrote:
>>> I'm wondering how we could use the perf to implament a clock_gettime.
>>> reading the perf fd or using ioctl is slow so reading the mmap
>>> ringbuffer is the only option. But as far as I know the ringbuffer has
>>> data only when an event is generated. Between two events, there is
>>> nothing we can read from the ringbuffer. Then how can application get
>>> time info in the interval?
>>
>> Are you wanting to read perf_clock from userspace?
>
> Yep, in some sort of form. Basically I want to read the time a task
> runs. Peter suggests we can read the activation time of a perf event.
> But I don't want to use any system call, as it's slow and likes
> clock_gettime.

Since we cannot get the capability committed upstream a number of folks 
are using this method:

     https://github.com/dsahern/linux/blob/perf-full-monty/README.ahern

ie., a KLM exports perf_clock and apps can use:

#define CLOCK_PERF 14
if (clock_gettime(CLOCK_PERF, &ts) != 0) {
}

No vdso acceleration, but works with an unmodified kernel.

David
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