cpuidle-pseries: Fix CEDE latency conversion from tb to us

Message ID 1598969293-29228-1-git-send-email-ego@linux.vnet.ibm.com
State New
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Series
  • cpuidle-pseries: Fix CEDE latency conversion from tb to us
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Commit Message

Gautham R. Shenoy Sept. 1, 2020, 2:08 p.m.
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>
---
 drivers/cpuidle/cpuidle-pseries.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Gautham R. Shenoy Sept. 2, 2020, 8:35 a.m. | #1
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
> >

Patch

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;