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

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Date:	Fri, 02 Jan 2015 10:26:10 -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 10:17 AM, Shaohua Li wrote:
> On Fri, Jan 02, 2015 at 10:09:10AM -0700, David Ahern wrote:
>> 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.
>
> no, that's not what I want. as I said, we don't want any syscall (unless
> it's vdso based).

A number of clocks are vdso based (e.g., CLOCK_MONOTONIC). Those are not 
good enough?

David
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