Re: [PATCH] sched_ext: Fix incorrect time delta calculation in time_delta()

[Andrea Righi]

On Sun, Feb 02, 2025 at 12:37:48PM +0900, Changwoo Min wrote:
> When (s64)(after - before) > 0, the code returns the result of
> (s64)(after - before) > 0 while the intended result should be
> (s64)(after - before). That happens because the middle operand of
> the ternary operator was omitted incorrectly, returning the result of
> (s64)(after - before) > 0. Thus, add the middle operand
> -- (s64)(after - before) -- to return the corRECT time calculation.
> 
> Fixes: d07be814fc71 ("sched_ext: Add time helpers for BPF schedulers")
> Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
> ---
>  tools/sched_ext/include/scx/common.bpf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
> index f254a39b86a5..d72b60a0c582 100644
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -432,7 +432,7 @@ void bpf_rcu_read_unlock(void) __ksym;
>   */
>  static inline s64 time_delta(u64 after, u64 before)
>  {
> -	return (s64)(after - before) > 0 ? : 0;
> +	return (s64)(after - before) > 0 ? (s64)(after - before) : 0;

Ugh.. how did we miss this? Thanks for noticing.

Acked-by: Andrea Righi <arighi@xxxxxxxxxx>

-Andrea