[tip: sched/urgent] sched/fair: Fix util_est UTIL_AVG_UNCHANGED handling

From: tip-bot2 for Dietmar Eggemann
Date: Fri Jun 04 2021 - 09:38:53 EST

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: 68d7a190682aa4eb02db477328088ebad15acc83
Gitweb: https://git.kernel.org/tip/68d7a190682aa4eb02db477328088ebad15acc83
Author: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
AuthorDate: Wed, 02 Jun 2021 16:58:08 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 03 Jun 2021 15:47:23 +02:00

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

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@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
Reviewed-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210602145808.1562603-1-dietmar.eggemann@xxxxxxx
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 d2c8813..28a98fc 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_AVG_UNCHANGED 0x80000000
} __attribute__((__aligned__(sizeof(u64))));

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 9c882f2..c5aacbd 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_est.enqueued);
+ PM(se.avg.util_est.enqueued, ~UTIL_AVG_UNCHANGED);
__PS("uclamp.min", p->uclamp_req[UCLAMP_MIN].value);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7b98fb3..2c8a935 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3907,7 +3907,7 @@ 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)
@@ -4007,7 +4007,7 @@ 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;
@@ -4056,6 +4056,7 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
ue.ewma += last_ewma_diff;
+ ue.enqueued |= UTIL_AVG_UNCHANGED;
WRITE_ONCE(p->se.avg.util_est, ue);

diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
index 1462846..cfe94ff 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).
- */
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))

- /* 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))