@@ -145,7 +145,20 @@ 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 NO_HZ 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.
+ */
+ idle_time = cur_idle_time > j_cdbs->prev_cpu_idle ?
+ cur_idle_time - j_cdbs->prev_cpu_idle : 0;
j_cdbs->prev_cpu_idle = cur_idle_time;
if (ignore_nice) {
@@ -162,7 +175,7 @@ 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 +202,13 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
load = j_cdbs->prev_load;
j_cdbs->prev_load = 0;
} else {
- 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;
- }
+ load = time_elapsed > idle_time ?
+ 100 * (time_elapsed - idle_time) / time_elapsed :
+ 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)
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> --- v2: - Avoid type conversion, compare current and previous idle time before obtaining 'idle_time'. - Update the explanation in comments. Discussions: v1: https://lore.kernel.org/linux-pm/20250210130659.3533182-1-zhanjie9@hisilicon.com/ --- drivers/cpufreq/cpufreq_governor.c | 42 ++++++++++++++---------------- 1 file changed, 19 insertions(+), 23 deletions(-)