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Message-Id: <20210602145808.1562603-1-dietmar.eggemann@arm.com>
Date: Wed, 2 Jun 2021 16:58:08 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Xuewen Yan <xuewen.yan94@...il.com>,
Vincent Donnefort <vincent.donnefort@....com>
Cc: Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Quentin Perret <qperret@...gle.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH v2] sched/fair: Fix util_est UTIL_AVG_UNCHANGED handling
The util_est internal UTIL_AVG_UNCHANGED flag which is used to prevent
unnecessary util_est updates uses the LSB of util_est.enqueued. It is
exposed via _task_util_est() (and task_util_est()).
Commit 92a801e5d5b7 ("sched/fair: Mask UTIL_AVG_UNCHANGED usages")
mentions that the LSB is lost for util_est resolution but
find_energy_efficient_cpu() checks if task_util_est() returns 0 to
return prev_cpu early.
_task_util_est() returns the max value of util_est.ewma and
util_est.enqueued or'ed w/ UTIL_AVG_UNCHANGED.
So task_util_est() returning the max of task_util() and
_task_util_est() will never return 0 under the default
SCHED_FEAT(UTIL_EST, true).
To fix this use the MSB of util_est.enqueued instead and keep the flag
util_est internal, i.e. don't export it via _task_util_est().
The maximal possible util_avg value for a task is 1024 so the MSB of
'unsigned int util_est.enqueued' isn't used to store a util value.
As a caveat the code behind the util_est_se trace point has to filter
UTIL_AVG_UNCHANGED to see the real util_est.enqueued value which should
be easy to do.
This also fixes an issue report by Xuewen Yan that util_est_update()
only used UTIL_AVG_UNCHANGED for the subtrahend of the equation:
last_enqueued_diff = ue.enqueued - (task_util() | UTIL_AVG_UNCHANGED)
Fixes: b89997aa88f0b sched/pelt: Fix task util_est update filtering
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
v1->v2:
1) Move UTIL_AVG_UNCHANGED definition into struct util_est.
2) Hide UTIL_AVG_UNCHANGED in proc_sched_show_task().
include/linux/sched.h | 8 ++++++++
kernel/sched/debug.c | 3 ++-
kernel/sched/fair.c | 5 +++--
kernel/sched/pelt.h | 11 +----------
4 files changed, 14 insertions(+), 13 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index c7e7d50e2fdc..ac5a7d29fd4f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -350,11 +350,19 @@ struct load_weight {
* Only for tasks we track a moving average of the past instantaneous
* estimated utilization. This allows to absorb sporadic drops in utilization
* of an otherwise almost periodic task.
+ *
+ * The UTIL_AVG_UNCHANGED flag is used to synchronize util_est with util_avg
+ * updates. When a task is dequeued, its util_est should not be updated if its
+ * util_avg has not been updated in the meantime.
+ * This information is mapped into the MSB bit of util_est.enqueued at dequeue
+ * time. Since max value of util_est.enqueued for a task is 1024 (PELT util_avg
+ * for a task) it is safe to use MSB.
*/
struct util_est {
unsigned int enqueued;
unsigned int ewma;
#define UTIL_EST_WEIGHT_SHIFT 2
+#define UTIL_AVG_UNCHANGED 0x80000000
} __attribute__((__aligned__(sizeof(u64))));
/*
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 3bdee5fd7d29..0c5ec2776ddf 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -885,6 +885,7 @@ static const struct seq_operations sched_debug_sops = {
#define __PS(S, F) SEQ_printf(m, "%-45s:%21Ld\n", S, (long long)(F))
#define __P(F) __PS(#F, F)
#define P(F) __PS(#F, p->F)
+#define PM(F, M) __PS(#F, p->F & (M))
#define __PSN(S, F) SEQ_printf(m, "%-45s:%14Ld.%06ld\n", S, SPLIT_NS((long long)(F)))
#define __PN(F) __PSN(#F, F)
#define PN(F) __PSN(#F, p->F)
@@ -1011,7 +1012,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
P(se.avg.util_avg);
P(se.avg.last_update_time);
P(se.avg.util_est.ewma);
- P(se.avg.util_est.enqueued);
+ PM(se.avg.util_est.enqueued, ~UTIL_AVG_UNCHANGED);
#endif
#ifdef CONFIG_UCLAMP_TASK
__PS("uclamp.min", p->uclamp_req[UCLAMP_MIN].value);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a2c30e52de76..56747df3f796 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3856,7 +3856,7 @@ static inline unsigned long _task_util_est(struct task_struct *p)
{
struct util_est ue = READ_ONCE(p->se.avg.util_est);
- return (max(ue.ewma, ue.enqueued) | UTIL_AVG_UNCHANGED);
+ return max(ue.ewma, (ue.enqueued & ~UTIL_AVG_UNCHANGED));
}
static inline unsigned long task_util_est(struct task_struct *p)
@@ -3956,7 +3956,7 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
* Reset EWMA on utilization increases, the moving average is used only
* to smooth utilization decreases.
*/
- ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
+ ue.enqueued = task_util(p);
if (sched_feat(UTIL_EST_FASTUP)) {
if (ue.ewma < ue.enqueued) {
ue.ewma = ue.enqueued;
@@ -4005,6 +4005,7 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
ue.ewma += last_ewma_diff;
ue.ewma >>= UTIL_EST_WEIGHT_SHIFT;
done:
+ ue.enqueued |= UTIL_AVG_UNCHANGED;
WRITE_ONCE(p->se.avg.util_est, ue);
trace_sched_util_est_se_tp(&p->se);
diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
index 9ed6d8c414ad..e06071bf3472 100644
--- a/kernel/sched/pelt.h
+++ b/kernel/sched/pelt.h
@@ -42,15 +42,6 @@ static inline u32 get_pelt_divider(struct sched_avg *avg)
return LOAD_AVG_MAX - 1024 + avg->period_contrib;
}
-/*
- * When a task is dequeued, its estimated utilization should not be update if
- * its util_avg has not been updated at least once.
- * This flag is used to synchronize util_avg updates with util_est updates.
- * We map this information into the LSB bit of the utilization saved at
- * dequeue time (i.e. util_est.dequeued).
- */
-#define UTIL_AVG_UNCHANGED 0x1
-
static inline void cfs_se_util_change(struct sched_avg *avg)
{
unsigned int enqueued;
@@ -58,7 +49,7 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
if (!sched_feat(UTIL_EST))
return;
- /* Avoid store if the flag has been already set */
+ /* Avoid store if the flag has been already reset */
enqueued = avg->util_est.enqueued;
if (!(enqueued & UTIL_AVG_UNCHANGED))
return;
--
2.25.1
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