On Fri, Sep 06, 2024 at 12:49:39PM +0200, Stefano Brivio wrote:
> If the nanoseconds of the minuend timestamp are less than the
> nanoseconds of the subtrahend timestamp, we need to carry one second
> in the subtraction.
> 
> I subtracted this second from the minuend, but didn't actually carry
> it in the subtraction of nanoseconds, and logged timestamps would jump
> back whenever we switched to the first branch of timespec_diff_us()
> from the second one.

Perhaps more to the point, the subtraction was the wrong way around
before.  I think we're too used to math mod 2^n, where reversing the
order of the arguments would be equivalent to adding an extra 2^n.

> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  util.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/util.c b/util.c
> index 6e64279..eede4e5 100644
> --- a/util.c
> +++ b/util.c
> @@ -249,7 +249,7 @@ void sock_probe_mem(struct ctx *c)
>  int64_t timespec_diff_us(const struct timespec *a, const struct timespec *b)
>  {
>  	if (a->tv_nsec < b->tv_nsec) {
> -		return (b->tv_nsec - a->tv_nsec) / 1000 +
> +		return (a->tv_nsec + 1000000000 - b->tv_nsec) / 1000 +
>  		       (a->tv_sec - b->tv_sec - 1) * 1000000;
>  	}
>  

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
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