Messages in this thread Patch in this message | | | From | Dietmar Eggemann <> | Subject | [PATCH] sched/fair: Fix util_est UTIL_AVG_UNCHANGED handling | Date | Fri, 14 May 2021 12:37:48 +0200 |
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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@arm.com> --- kernel/sched/fair.c | 5 +++-- kernel/sched/pelt.h | 13 +++++++------ 2 files changed, 10 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 161b92aa1c79..0150d440b0a2 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..178290a8d150 100644 --- a/kernel/sched/pelt.h +++ b/kernel/sched/pelt.h @@ -43,13 +43,14 @@ static inline u32 get_pelt_divider(struct sched_avg *avg) } /* - * When a task is dequeued, its estimated utilization should not be update if - * its util_avg has not been updated at least once. + * When a task is dequeued, its estimated utilization should not be updated if + * its util_avg has not been updated in the meantime. * 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). + * We map this information 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 here. */ -#define UTIL_AVG_UNCHANGED 0x1 +#define UTIL_AVG_UNCHANGED 0x80000000 static inline void cfs_se_util_change(struct sched_avg *avg) { @@ -58,7 +59,7 @@ 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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