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

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hello Joel,

On Wed, Sep 02, 2020 at 01:08:35AM +0000, Joel Stanley wrote:
> On Tue, 1 Sep 2020 at 14:09, Gautham R. Shenoy <ego@linux.vnet.ibm.com> wrote:
> >
> > From: "Gautham R. Shenoy" <ego@linux.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@linux.vnet.ibm.com>
> 
> Reviewed-by: Joel Stanley <joel@jms.id.au>
>

Thanks for reviewing the fix.

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

Yes, that would be better. I will post a v2 with that.

> 
> > ---
> >  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
> >