Re: [PATCH] cpuidle-pseries: Fix CEDE latency conversion from tb to us

[Joel Stanley]

On Tue, 1 Sep 2020 at 14:09, Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx> wrote:
>
> From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>
>
> commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
> CEDE(0)") sets the exit latency of CEDE(0) based on the latency values
> of the Extended CEDE states advertised by the platform. The values
> advertised by the platform are in timebase ticks. However the cpuidle
> framework requires the latency values in microseconds.
>
> If the tb-ticks value advertised by the platform correspond to a value
> smaller than 1us, during the conversion from tb-ticks to microseconds,
> in the current code, the result becomes zero. This is incorrect as it
> puts a CEDE state on par with the snooze state.
>
> This patch fixes this by rounding up the result obtained while
> converting the latency value from tb-ticks to microseconds.
>
> Fixes: commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
> CEDE(0)")
>
> Signed-off-by: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Joel Stanley <joel@xxxxxxxxx>

Should you check for the zero case and print a warning?

> ---
>  drivers/cpuidle/cpuidle-pseries.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
> index ff6d99e..9043358 100644
> --- a/drivers/cpuidle/cpuidle-pseries.c
> +++ b/drivers/cpuidle/cpuidle-pseries.c
> @@ -361,7 +361,7 @@ static void __init fixup_cede0_latency(void)
>         for (i = 0; i < nr_xcede_records; i++) {
>                 struct xcede_latency_record *record = &payload->records[i];
>                 u64 latency_tb = be64_to_cpu(record->latency_ticks);
> -               u64 latency_us = tb_to_ns(latency_tb) / NSEC_PER_USEC;
> +               u64 latency_us = DIV_ROUND_UP_ULL(tb_to_ns(latency_tb), NSEC_PER_USEC);
>
>                 if (latency_us < min_latency_us)
>                         min_latency_us = latency_us;
> --
> 1.9.4
>