Re: [PATCH v5] perf: Use monotonic clock as a source for timestamps

[John Stultz]

On Thu, Feb 12, 2015 at 11:38 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Feb 12, 2015 at 11:28:14AM +0100, Peter Zijlstra wrote:
>> > and you would have to check the clocksource is TSC.
>>
>> It implicitly does that; it has that sched_clock_stable() thing, but
>> yeah I suppose someone could change the clocksource even though the tsc
>> is stable.
>>
>> Not using TSC when its available is quite crazy though.. but sure.
>
> Something like this on top then.. it might have a few header issues, the
> whole asm/tsc.h vs clocksource.h thing looks like pain.
>
> I haven't tried to compile it, maybe we can move cycle_t into types and
> fwd declare struct clocksource or whatnot.
>
> Of course, all this is quite horrible on the timekeeping side; it might
> be tglx and/or jstutlz are having spasms just reading it :-)


Oof.. Yea, this exposes all sorts of timekeeping internals out to the
rest of the kernel that I'd rather not have out there.

> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1967,17 +1967,19 @@ static void local_clock_user_time(struct
>         cyc2ns_read_end(data);
>  }
>
> -extern void notrace __ktime_get_mono_fast(u64 *offset, u32 *mult, u16 *shift);
> +extern bool notrace __ktime_get_mono_fast(cycle_t (*read)(struct clocksource *cs),
> +               u64 *offset, u32 *mult, u16 *shift);
>
>  static void ktime_fast_mono_user_time(struct perf_event_mmap_page *userpg, u64 now)
>  {
> +       if (!__ktime_get_mono_fast(read_tsc, &userpg->time_zero,
> +                                  &userpg->time_mult,
> +                                  &userpg->time_shift))

Soo.. instead of hard-coding read_tsc here, can we instead use a
clocksource flag that we can check that the current clocksource is
valid for this sort of use?

thanks
-john
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