Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count

[John Stultz]

On Wed, Nov 16, 2016 at 11:35 AM, Chris Metcalf <cmetcalf@xxxxxxxxxxxx> wrote:
> For large values of "mult" and long uptimes, the intermediate
> result of "cycles * mult" can overflow 64 bits.  For example,
> the tile platform calls clocksource_cyc2ns with a 1.2 GHz clock;
> we have mult = 853, and after 208.5 days, we overflow 64 bits.
>
> Since clocksource_cyc2ns() is intended to be used for relative
> cycle counts, not absolute cycle counts, performance is more
> importance than accepting a wider range of cycle values.
> So, just use mult_frac() directly in tile's sched_clock().
>
> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> ---
> Blackfin should make a similar change in their sched_clock().
>
>  arch/tile/kernel/time.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
> index 178989e6d3e3..ea960d660917 100644
> --- a/arch/tile/kernel/time.c
> +++ b/arch/tile/kernel/time.c
> @@ -218,8 +218,8 @@ void do_timer_interrupt(struct pt_regs *regs, int fault_num)
>   */
>  unsigned long long sched_clock(void)
>  {
> -       return clocksource_cyc2ns(get_cycles(),
> -                                 sched_clock_mult, SCHED_CLOCK_SHIFT);
> +       return mult_frac(get_cycles(),
> +                        sched_clock_mult, 1ULL << SCHED_CLOCK_SHIFT);
>  }

So... looking closer at mult_frac(), its a really slow implementation,
doing 2 divs and a mod and a mult. Hopefully the compiler can sort out
the divs are power of two, and optimize it out, but I'm still
hesitant.

sched_clock() is normally a very hot-path call, so this might have a
real performance impact, especially compared to what its replacing.

In your earlier patch, you mentioned this was similar to 4cecf6d401a0
("sched, x86: Avoid unnecessary overflow in
sched_clock"). It might be better to actually try to use similar logic
there, to make sure the performance impact is minimal.

thanks
-john