linux-kernel - [PATCH v2] sched/pelt: Fix task util

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20210225165820.1377125-1-vincent.donnefort@arm.com>
Date:   Thu, 25 Feb 2021 16:58:20 +0000
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     peterz@...radead.org, mingo@...dhat.com, vincent.guittot@...aro.org
Cc:     dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        patrick.bellasi@...bug.net, valentin.schneider@....com,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH v2] sched/pelt: Fix task util_est update filtering

Being called for each dequeue, util_est reduces the number of its updates
by filtering out when the EWMA signal is different from the task util_avg
by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the
decay from a previous high util_avg, EWMA might now be close enough to
the new util_avg. No update would then happen while it would leave
ue.enqueued with an out-of-date value.

Taking into consideration the two util_est members, EWMA and enqueued for
the filtering, ensures, for both, an up-to-date value.

This is for now an issue only for the trace probe that might return the
stale value. Functional-wise, it isn't a problem, as the value is always
accessed through max(enqueued, ewma).

This problem has been observed using LISA's UtilConvergence:test_means on
the sd845c board.

No regression observed with Hackbench on sd845c and Perf-bench sched pipe
on hikey/hikey960.

Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e4104ae39ae..214e02862994 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3966,24 +3966,27 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
 	trace_sched_util_est_cfs_tp(cfs_rq);
 }
 
+#define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100)
+
 /*
- * Check if a (signed) value is within a specified (unsigned) margin,
+ * Check if a (signed) value is within the (unsigned) util_est margin,
  * based on the observation that:
  *
  *     abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
  *
- * NOTE: this only works when value + maring < INT_MAX.
+ * NOTE: this only works when value + UTIL_EST_MARGIN < INT_MAX.
  */
-static inline bool within_margin(int value, int margin)
+static inline bool util_est_within_margin(int value)
 {
-	return ((unsigned int)(value + margin - 1) < (2 * margin - 1));
+	return ((unsigned int)(value + UTIL_EST_MARGIN - 1) <
+		(2 * UTIL_EST_MARGIN - 1));
 }
 
 static inline void util_est_update(struct cfs_rq *cfs_rq,
 				   struct task_struct *p,
 				   bool task_sleep)
 {
-	long last_ewma_diff;
+	long last_ewma_diff, last_enqueued_diff;
 	struct util_est ue;
 
 	if (!sched_feat(UTIL_EST))
@@ -4004,6 +4007,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
 	if (ue.enqueued & UTIL_AVG_UNCHANGED)
 		return;
 
+	last_enqueued_diff = ue.enqueued;
+
 	/*
 	 * Reset EWMA on utilization increases, the moving average is used only
 	 * to smooth utilization decreases.
@@ -4017,12 +4022,17 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
 	}
 
 	/*
-	 * Skip update of task's estimated utilization when its EWMA is
+	 * Skip update of task's estimated utilization when its members are
 	 * already ~1% close to its last activation value.
 	 */
 	last_ewma_diff = ue.enqueued - ue.ewma;
-	if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
+	last_enqueued_diff -= ue.enqueued;
+	if (util_est_within_margin(last_ewma_diff)) {
+		if (!util_est_within_margin(last_enqueued_diff))
+			goto done;
+
 		return;
+	}
 
 	/*
 	 * To avoid overestimation of actual task utilization, skip updates if
-- 
2.25.1