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

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Message-ID: <CACPK8XfZdnKusEuu8i=-aH=Wfr6X6sMrvX=btFq9PtnXJ2w-SQ@mail.gmail.com>
Date:   Wed, 2 Sep 2020 01:08:35 +0000
From:   Joel Stanley <joel@....id.au>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpuidle-pseries: Fix CEDE latency conversion from tb to us

On Tue, 1 Sep 2020 at 14:09, Gautham R. Shenoy <ego@...ux.vnet.ibm.com> wrote:
>
> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>
> 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@...ux.vnet.ibm.com>

Reviewed-by: Joel Stanley <joel@....id.au>

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
>