[PATCH] cpuidle/pseries: Fixup CEDE0 latency only for POWER10 onwards

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

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

On some of the POWER9 LPARs, the older firmwares advertise a very low
value of 2us for CEDE1 exit latency on a Dedicated LPAR. However the
measured value is 5us on an average. Due to the low advertised exit
latency, we are entering CEDE(0) more aggressively on such
platforms. While this helps achieve SMT folding faster, we pay the
penalty of having to send an IPI to wakeup the CPU when the target
residency is very short. This is showing up as a performance
regression on the newer kernels running on the LPARs with older
firmware.

Hence, set the exit latency of CEDE(0) based on the latency values
advertized by platform only from POWER10 onwards. The values
advertized on POWER10 platforms is more realistic and informed by the
latency measurements.

For platforms older than POWER10, retain the hardcoded value of exit
latency, which is 10us. Though this is higher than the measured
values, we would be erring on the side of caution.

Reported-by: Enrico Joedecke <joedecke@de.ibm.com>
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 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index a2b5c6f..7207467 100644
--- a/drivers/cpuidle/cpuidle-pseries.c
+++ b/drivers/cpuidle/cpuidle-pseries.c
@@ -419,7 +419,8 @@ static int pseries_idle_probe(void)
 			cpuidle_state_table = shared_states;
 			max_idle_state = ARRAY_SIZE(shared_states);
 		} else {
-			fixup_cede0_latency();
+			if (pvr_version_is(PVR_POWER10))
+				fixup_cede0_latency();
 			cpuidle_state_table = dedicated_states;
 			max_idle_state = NR_DEDICATED_STATES;
 		}
-- 
1.9.4