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

[Shaohua Li <shli@fb.com>]

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). 

Thanks,
Shaohua