[PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

[PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

[Gautham R. Shenoy]

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. It also
prints a warning in case we discover an extended-cede state with
wakeup latency to be 0. In such a case, ensure that CEDE(0) has a
non-zero wakeup latency.

Fixes: commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
CEDE(0)")

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---
v1-->v2: Added a warning if a CEDE state has 0 wakeup latency (Suggested by Joel Stanley)
         Also added code to ensure that CEDE(0) has a non-zero wakeup latency.	 
 drivers/cpuidle/cpuidle-pseries.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index ff6d99e..a2b5c6f 100644
--- a/drivers/cpuidle/cpuidle-pseries.c
+++ b/drivers/cpuidle/cpuidle-pseries.c
@@ -361,7 +361,10 @@ 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 == 0)
+			pr_warn("cpuidle: xcede record %d has an unrealistic latency of 0us.\n", i);
 
 		if (latency_us < min_latency_us)
 			min_latency_us = latency_us;
@@ -378,10 +381,14 @@ static void __init fixup_cede0_latency(void)
 	 * Perform the fix-up.
 	 */
 	if (min_latency_us < dedicated_states[1].exit_latency) {
-		u64 cede0_latency = min_latency_us - 1;
+		/*
+		 * We set a minimum of 1us wakeup latency for cede0 to
+		 * distinguish it from snooze
+		 */
+		u64 cede0_latency = 1;
 
-		if (cede0_latency <= 0)
-			cede0_latency = min_latency_us;
+		if (min_latency_us > cede0_latency)
+			cede0_latency = min_latency_us - 1;
 
 		dedicated_states[1].exit_latency = cede0_latency;
 		dedicated_states[1].target_residency = 10 * (cede0_latency);
-- 
1.9.4

Re: [PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

[Vaidyanathan Srinivasan]

* Gautham R Shenoy <ego@linux.vnet.ibm.com> [2020-09-03 14:57:27]:

> 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. It also
> prints a warning in case we discover an extended-cede state with
> wakeup latency to be 0. In such a case, ensure that CEDE(0) has a
> non-zero wakeup latency.
> 
> Fixes: commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
> CEDE(0)")
> 
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>

Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>

> ---
> v1-->v2: Added a warning if a CEDE state has 0 wakeup latency (Suggested by Joel Stanley)
>          Also added code to ensure that CEDE(0) has a non-zero wakeup latency.	 
>  drivers/cpuidle/cpuidle-pseries.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
> index ff6d99e..a2b5c6f 100644
> --- a/drivers/cpuidle/cpuidle-pseries.c
> +++ b/drivers/cpuidle/cpuidle-pseries.c
> @@ -361,7 +361,10 @@ 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);

This would fix the issue of rounding down to zero.

> +
> +		if (latency_us == 0)
> +			pr_warn("cpuidle: xcede record %d has an unrealistic latency of 0us.\n", i);

+1  This should not happen.

>  		if (latency_us < min_latency_us)
>  			min_latency_us = latency_us;
> @@ -378,10 +381,14 @@ static void __init fixup_cede0_latency(void)
>  	 * Perform the fix-up.
>  	 */
>  	if (min_latency_us < dedicated_states[1].exit_latency) {
> -		u64 cede0_latency = min_latency_us - 1;
> +		/*
> +		 * We set a minimum of 1us wakeup latency for cede0 to
> +		 * distinguish it from snooze
> +		 */
> +		u64 cede0_latency = 1;
> 
> -		if (cede0_latency <= 0)
> -			cede0_latency = min_latency_us;
> +		if (min_latency_us > cede0_latency)
> +			cede0_latency = min_latency_us - 1;

Good checks to expect cede latency of 1us or more.

>  		dedicated_states[1].exit_latency = cede0_latency;
>  		dedicated_states[1].target_residency = 10 * (cede0_latency);

--Vaidy

Re: [PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

[Michael Ellerman]

On Thu, 3 Sep 2020 14:57:27 +0530, Gautham R. Shenoy wrote:
> 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.
> 
> [...]

Applied to powerpc/fixes.

[1/1] cpuidle: pseries: Fix CEDE latency conversion from tb to us
      https://git.kernel.org/powerpc/c/1d3ee7df009a46440c58508b8819213c09503acd

cheers