[PATCH 3/3] pmdomain: Extend the genpd governor for CPUs to account for IPIs

Ulf Hansson
2025-10-03T15:03:16Z

When the genpd governor for CPUs, tries to select the most optimal
idlestate for a group of CPUs managed in a PM domain, it fails far too
often.

On a Dragonboard 410c, which is an arm64 based platform with 4 CPUs
in one cluster that is using PSCI OS-initiated mode, we can observe that we
often fail when trying to enter the selected idlestate. This is certainly a
suboptimal behaviour that leads to many unnecessary requests being sent to
the PSCI FW.

A simple dd operation that reads from the eMMC, to generate some IRQs and
I/O handling helps us to understand the problem, while also monitoring the
rejected counters in debugfs for the corresponding idlestates of the genpd
in question.

Menu governor:
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 1451 437 91 149 0
S1 65194 558 149 172 0
dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500
524288000 bytes (500.0MB) copied, 3.562698 seconds, 140.3MB/s
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 2694 1073 265 892 1
S1 74567 829 561 790 0

The dd completed in ~3.6 seconds and rejects increased with 586.

Teo governor:
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 4976 2096 392 1721 2
S1 160661 1893 1309 1904 0
dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500
524288000 bytes (500.0MB) copied, 3.543225 seconds, 141.1MB/s
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 5192 2194 433 1830 2
S1 167677 2891 3184 4729 0

The dd completed in ~3.6 seconds and rejects increased with 1916.

The main reason to the above problem is pending IPIs for one of the CPUs
that is affected by the idlestate that the genpd governor selected. This
leads to that the PSCI FW refuses to enter it. To improve the behaviour,
let's start to take into account pending IPIs for CPUs in the genpd
governor, hence we fallback to use the shallower per CPU idlestate.

Re-testing with this change shows a significant improved behaviour.

- Menu governor:
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 1994 551 10 24 0
S1 115602 801 4 56 0
dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500
524288000 bytes (500.0MB) copied, 3.622631 seconds, 138.0MB/s
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 2462 766 14 202 0
S1 119559 1031 9 253 0

The dd completed in ~3.6 seconds and rejects increased with 9.

- Teo governor
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 3212 990 16 245 0
S1 202442 2459 13 1184 0
dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500
524288000 bytes (500.0MB) copied, 3.284563 seconds, 152.2MB/s
cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states
State Time Spent(ms) Usage Rejected Above Below
S0 3387 1046 16 265 0
S1 206074 2826 19 1524 0

The dd completed in ~3.3 seconds and rejects increased with 6.

Note that, the rejected counters in genpd are also being accumulated in the
rejected counters that are managed by cpuidle, yet on a per CPU idlestates
basis. Comparing these counters before/after this change, through cpuidle's
sysfs interface shows the similar improvements.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
---
drivers/pmdomain/governor.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/pmdomain/governor.c b/drivers/pmdomain/governor.c
index 39359811a930..7e81dc383269 100644
--- a/drivers/pmdomain/governor.c
+++ b/drivers/pmdomain/governor.c
@@ -404,15 +404,21 @@ static bool cpu_power_down_ok(struct dev_pm_domain *pd)
if ((idle_duration_ns >= (genpd->states[i].residency_ns +
genpd->states[i].power_off_latency_ns)) &&
(global_constraint >= (genpd->states[i].power_on_latency_ns +
- genpd->states[i].power_off_latency_ns))) {
- genpd->state_idx = i;
- genpd->gd->last_enter = now;
- genpd->gd->reflect_residency = true;
- return true;
- }
+ genpd->states[i].power_off_latency_ns)))
+ break;
+
} while (--i >= 0);

- return false;
+ if (i < 0)
+ return false;
+
+ if (cpus_has_pending_ipi(genpd->cpus))
+ return false;
+
+ genpd->state_idx = i;
+ genpd->gd->last_enter = now;
+ genpd->gd->reflect_residency = true;
+ return true;
}

struct dev_power_governor pm_domain_cpu_gov = {
--
2.43.0