[PATCH 5.4 081/154] cpufreq: governor: Fix negative idle_time handling in dbs_update()

Greg Kroah-Hartman
2025-04-08T12:05:48Z

From: Jie Zhan <zhanjie9@hisilicon.com>

[ Upstream commit 3698dd6b139dc37b35a9ad83d9330c1f99666c02 ]

We observed an issue that the CPU frequency can't raise up with a 100% CPU
load when NOHZ is off and the 'conservative' governor is selected.

'idle_time' can be negative if it's obtained from get_cpu_idle_time_jiffy()
when NOHZ is off. This was found and explained in commit 9485e4ca0b48
("cpufreq: governor: Fix handling of special cases in dbs_update()").

However, commit 7592019634f8 ("cpufreq: governors: Fix long idle detection
logic in load calculation") introduced a comparison between 'idle_time' and
'samling_rate' to detect a long idle interval. While 'idle_time' is
converted to int before comparison, it's actually promoted to unsigned
again when compared with an unsigned 'sampling_rate'. Hence, this leads to
wrong idle interval detection when it's in fact 100% busy and sets
policy_dbs->idle_periods to a very large value. 'conservative' adjusts the
frequency to minimum because of the large 'idle_periods', such that the
frequency can't raise up. 'Ondemand' doesn't use policy_dbs->idle_periods
so it fortunately avoids the issue.

Correct negative 'idle_time' to 0 before any use of it in dbs_update().

Fixes: 7592019634f8 ("cpufreq: governors: Fix long idle detection logic in load calculation")
Signed-off-by: Jie Zhan <zhanjie9@hisilicon.com>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Link: https://patch.msgid.link/20250213035510.2402076-1-zhanjie9@hisilicon.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/cpufreq/cpufreq_governor.c | 45 +++++++++++++++---------------
1 file changed, 23 insertions(+), 22 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 4bb054d0cb432..01d19d9ac0fe8 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -145,7 +145,23 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
time_elapsed = update_time - j_cdbs->prev_update_time;
j_cdbs->prev_update_time = update_time;

- idle_time = cur_idle_time - j_cdbs->prev_cpu_idle;
+ /*
+ * cur_idle_time could be smaller than j_cdbs->prev_cpu_idle if
+ * it's obtained from get_cpu_idle_time_jiffy() when NOHZ is
+ * off, where idle_time is calculated by the difference between
+ * time elapsed in jiffies and "busy time" obtained from CPU
+ * statistics. If a CPU is 100% busy, the time elapsed and busy
+ * time should grow with the same amount in two consecutive
+ * samples, but in practice there could be a tiny difference,
+ * making the accumulated idle time decrease sometimes. Hence,
+ * in this case, idle_time should be regarded as 0 in order to
+ * make the further process correct.
+ */
+ if (cur_idle_time > j_cdbs->prev_cpu_idle)
+ idle_time = cur_idle_time - j_cdbs->prev_cpu_idle;
+ else
+ idle_time = 0;
+
j_cdbs->prev_cpu_idle = cur_idle_time;

if (ignore_nice) {
@@ -162,7 +178,7 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
* calls, so the previous load value can be used then.
*/
load = j_cdbs->prev_load;
- } else if (unlikely((int)idle_time > 2 * sampling_rate &&
+ } else if (unlikely(idle_time > 2 * sampling_rate &&
j_cdbs->prev_load)) {
/*
* If the CPU had gone completely idle and a task has
@@ -189,30 +205,15 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
load = j_cdbs->prev_load;
j_cdbs->prev_load = 0;
} else {
- if (time_elapsed >= idle_time) {
+ if (time_elapsed > idle_time)
load = 100 * (time_elapsed - idle_time) / time_elapsed;
- } else {
- /*
- * That can happen if idle_time is returned by
- * get_cpu_idle_time_jiffy(). In that case
- * idle_time is roughly equal to the difference
- * between time_elapsed and "busy time" obtained
- * from CPU statistics. Then, the "busy time"
- * can end up being greater than time_elapsed
- * (for example, if jiffies_64 and the CPU
- * statistics are updated by different CPUs),
- * so idle_time may in fact be negative. That
- * means, though, that the CPU was busy all
- * the time (on the rough average) during the
- * last sampling interval and 100 can be
- * returned as the load.
- */
- load = (int)idle_time < 0 ? 100 : 0;
- }
+ else
+ load = 0;
+
j_cdbs->prev_load = load;
}

- if (unlikely((int)idle_time > 2 * sampling_rate)) {
+ if (unlikely(idle_time > 2 * sampling_rate)) {
unsigned int periods = idle_time / sampling_rate;

if (periods < idle_periods)
--
2.39.5