Messages in this thread Patch in this message | | | From | Chen Yu <> | Subject | [RFC PATCH v4 1/2] sched/fair: Introduce short duration task check | Date | Fri, 16 Dec 2022 14:11:17 +0800 |
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Introduce short-duration task checks, as there is a requirement to leverage this attribute for better task placement.
There are several choices of metrics that could be used to indicate if a task is a short-duration task.
At first thought the (p->se.sum_exec_runtime / p->nvcsw) can be used to measure the task duration. However, the history long past was factored too heavily in such a formula. Ideally, the old activity should decay and not affect the current status too much.
Although something based on PELT can be used, se.util_avg might not be appropriate to describe the task duration: 1. Task p1 and task p2 are doing frequent ping-pong scheduling on one CPU, both p1 and p2 have a short duration, but the util_avg can be up to 50%. 2. Suppose a task lasting less than 4ms is regarded as a short task. If task p3 runs for 6 ms and sleeps for 32 ms, p3 should not be a short-duration task. However, PELT would decay p3's accumulated running time from 6 ms to 3 ms, because 32 ms is the half-life in PELT. As a result, p3 would be incorrectly treated as a short task.
It was found that there was once a similar feature to track the duration of a task, which is in Commit ad4b78bbcbab ("sched: Add new wakeup preemption mode: WAKEUP_RUNNING"). Unfortunately, it was reverted because it was an experiment. So pick the patch up again, by recording the average duration when a task voluntarily switches out.
The threshold of short duration is sysctl_sched_min_granularity / 8, so it can be tuned by the user. By default, the threshold is 375 us. The reason to reuse sysctl_sched_min_granularity is that it reflects how long the user would like the task to run. So the criteria of a short task have a connection to it.
Josh is not in favor of tying the threshold to sysctl_sched_min_granularity, ideally there should be a dedicated parameter for the threshold, but that introduces complexity for maintenance and the user.
Introduce SIS_SHORT to enable the short duration check.
Suggested-by: Tim Chen <tim.c.chen@intel.com> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Chen Yu <yu.c.chen@intel.com> --- include/linux/sched.h | 3 +++ kernel/sched/core.c | 2 ++ kernel/sched/debug.c | 1 + kernel/sched/fair.c | 23 +++++++++++++++++++++++ kernel/sched/features.h | 1 + 5 files changed, 30 insertions(+)
diff --git a/include/linux/sched.h b/include/linux/sched.h index ffb6eb55cd13..26f4768e63f3 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -557,6 +557,9 @@ struct sched_entity { u64 prev_sum_exec_runtime; u64 nr_migrations; + u64 prev_sum_exec_runtime_vol; + /* average duration of a task */ + u64 dur_avg; #ifdef CONFIG_FAIR_GROUP_SCHED int depth; diff --git a/kernel/sched/core.c b/kernel/sched/core.c index daff72f00385..c5202f1be3f7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4348,6 +4348,8 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p) p->se.prev_sum_exec_runtime = 0; p->se.nr_migrations = 0; p->se.vruntime = 0; + p->se.dur_avg = 0; + p->se.prev_sum_exec_runtime_vol = 0; INIT_LIST_HEAD(&p->se.group_node); #ifdef CONFIG_FAIR_GROUP_SCHED diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index 1637b65ba07a..8d64fba16cfe 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -1024,6 +1024,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns, __PS("nr_involuntary_switches", p->nivcsw); P(se.load.weight); + P(se.dur_avg); #ifdef CONFIG_SMP P(se.avg.load_sum); P(se.avg.runnable_sum); diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e4a0b8bd941c..abdb7a442052 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4824,6 +4824,16 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se) static int wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se); +/* + * If a task switches in and then voluntarily relinquishes the + * CPU quickly, it is regarded as a short duration task. + */ +static inline int is_short_task(struct task_struct *p) +{ + return sched_feat(SIS_SHORT) && p->se.dur_avg && + ((p->se.dur_avg * 8) <= sysctl_sched_min_granularity); +} + /* * Pick the next process, keeping these things in mind, in this order: * 1) keep things fair between processes/task groups @@ -5995,6 +6005,18 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) static void set_next_buddy(struct sched_entity *se); +static inline void dur_avg_update(struct task_struct *p, bool task_sleep) +{ + u64 dur; + + if (!task_sleep) + return; + + dur = p->se.sum_exec_runtime - p->se.prev_sum_exec_runtime_vol; + p->se.prev_sum_exec_runtime_vol = p->se.sum_exec_runtime; + update_avg(&p->se.dur_avg, dur); +} + /* * The dequeue_task method is called before nr_running is * decreased. We remove the task from the rbtree and @@ -6067,6 +6089,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags) dequeue_throttle: util_est_update(&rq->cfs, p, task_sleep); + dur_avg_update(p, task_sleep); hrtick_update(rq); } diff --git a/kernel/sched/features.h b/kernel/sched/features.h index ee7f23c76bd3..efdc29c42161 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -62,6 +62,7 @@ SCHED_FEAT(TTWU_QUEUE, true) */ SCHED_FEAT(SIS_PROP, false) SCHED_FEAT(SIS_UTIL, true) +SCHED_FEAT(SIS_SHORT, true) /* * Issue a WARN when we do multiple update_rq_clock() calls -- 2.25.1
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