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

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

On 04/03/2013 10:19 AM, Pawel Moll wrote:
> On Tue, 2013-04-02 at 17:19 +0100, John Stultz wrote:
>> But if we're going to have to do
>> this via a clockid, I'm going to want it to be done via a dynamic posix
>> clockid, so its clear its tightly tied with perf and not considered a
>> generic interface (and I can clearly point folks having problems to the
>> perf maintainers ;).
> Ok, so how about the code below?
>
> There are two distinct parts of the "solution":
>
> 1. The dynamic posix clock, as you suggested. Then one can get the perf
> timestamp by doing:
>
> 	clock_fd = open("/dev/perf-clock", O_RDONLY);
> 	clock_gettime(FD_TO_CLOCKID(clock_fd), &ts)
>
> 2. A sort-of-hack in the get_posix_clock() function making it possible
> to do the same using the perf event file descriptor, eg.:
>
> 	fd = sys_perf_event_open(&attr, -1, 0, -1, 0);
> 	clock_gettime(FD_TO_CLOCKID(fd), &ts)

#2 makes my nose wrinkle. Forgive me for being somewhat ignorant on the 
perf interfaces, but why is the second portion necessary or beneficial?

thanks
-john