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

[David Ahern <dsahern@gmail.com>]

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