cpufreq: governor: Fix negative 'idle_time' handling in dbs_update()

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 9485e4c
("cpufreq: governor: Fix handling of special cases in dbs_update()").

However, commit 7592019 ("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: 7592019 ("cpufreq: governors: Fix long idle detection logic in load calculation")
Signed-off-by: Jie Zhan <[email protected]>
Reviewed-by: Chen Yu <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Jie Zhan

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)