Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

[John Stultz <john.stultz@linaro.org>]

On 04/08/2013 10:58 AM, Pawel Moll wrote:
> Now, before I spend time doing all this, a question to John, Peter,
> Stephane and the rest of the public - would a solution providing such
> userspace interface:
>
> 	fd = sys_perf_open()
> 	timestamp = clock_gettime((FD_TO_CLOCKID(fd), &ts)
>
> be acceptable to all?

So thinking this through further, I'm worried we may _not_ be able to 
eventually enable this to be a vdso as I had earlier hoped. Mostly 
because I'm not sure how the fd -> file -> clock lookup could be done in 
userland (any ideas?).

So this makes this approach mostly equivalent long term to the ioctl 
method, from a performance perspective. And makes the dynamic posix 
clockid somewhat less of a middle-ground compromise between the ioctl 
and generic constant clockid approach.

So while I'm not opposed to the sort of extention proposed above, I want 
to make sure introducing the new approach is worth the effort when 
compared with just adding an ioctl.

thanks
-john