Subject: Re: [PATCH v2 3/7] timecounter: allow for non-power of two overflow

From: Thomas Gleixner <[email-masked]>
Date: Wed, 24 Nov 2021 21:55:44 +0100

Martin,

On Tue, Nov 09 2021 at 10:50, Martin Kaistra wrote:
>   *			see CYCLECOUNTER_MASK() helper macro
>   * @mult:		cycle to nanosecond multiplier
>   * @shift:		cycle to nanosecond divisor (power of two)
> + * @overflow_point:	non-power of two overflow point (optional),
> + *			smaller than mask
>   */
>  struct cyclecounter {
>  	u64 (*read)(const struct cyclecounter *cc);
>  	u64 mask;
>  	u32 mult;
>  	u32 shift;
> +	u64 overflow_point;
>  };
>  
>  /**
> diff --git a/kernel/time/timecounter.c b/kernel/time/timecounter.c
> index e6285288d765..afd2910a9724 100644
> --- a/kernel/time/timecounter.c
> +++ b/kernel/time/timecounter.c
> @@ -39,6 +39,9 @@ static u64 timecounter_read_delta(struct timecounter *tc)
>  	/* calculate the delta since the last timecounter_read_delta(): */
>  	cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
>  
> +	if (tc->cc->overflow_point && (cycle_now - tc->cycle_last) > tc->cc->mask)
> +		cycle_delta -= tc->cc->mask - tc->cc->overflow_point;

TBH, this took me more than one twisted braincell to grok.

With support for clocks which do not wrap at power of 2 boundaries we
already lose the unconditional fast path no matter what. So what's the
point of having two conditions and doing this convoluted math here?

In timecounter_init():

   	tc->ovfl = cc->ovfl ? cc->ovfl : cc->mask + 1;

which makes it a common path in timecounter_read_delta():

  	cycle_delta = cycle_now - tc->cycle_last;
        if ((s64)cycle_delta) < 0)
        	cycle_delta += tc->ovfl;

which produces way better binary code.

The conditional does not really matter for the timecounter use cases as
that calculation is noise compared to the actual cc->read() access.

Aside of that the same problem exists in timecounter_cyc2time()...

After that we probably should do a treewide sweep to get rid of cc->mask
to avoid confusion and subtle to understand errors when some code uses
cc->mask instead of cc->ovfl.

Thanks,

        tglx