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

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Message-ID: <20150102170223.GA2381637@devbig257.prn2.facebook.com>
Date:	Fri, 2 Jan 2015 09:02:23 -0800
From:	Shaohua Li <shli@...com>
To:	David Ahern <dsahern@...il.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 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.

Thanks,
Shaohua
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