Messages in this thread Patch in this message | | | From | Chen Yu <> | Subject | [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check | Date | Sun, 23 Oct 2022 23:32:31 +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) could 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.
PELT could decay the runtime time, so maybe something like se.util_avg could be used. But after a second thought, PELT might not be appropriate to measure the 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 6ms and sleeps for 32ms, it should not be a short-duration task. However, PELT would decay p3's accumulated running time from 6ms to 3ms, because 32ms 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 calculation of average duration is borrowed from the idea of Exponential Weighted Moving Average (EWMA) in util_est_update(). The effect is that the average task duration is calculated by: new_avg_duration = old_avg_duration * 0.875 + lastest_duration * 0.125; which is what update_avg() does now.
Introduce SIS_SHORT to control this strategy.
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 | 8 +++++ kernel/sched/core.c | 2 ++ kernel/sched/fair.c | 77 +++++++++++++++++++++++++++++++++++++++++ kernel/sched/features.h | 1 + 4 files changed, 88 insertions(+)
diff --git a/include/linux/sched.h b/include/linux/sched.h index ffb6eb55cd13..ff74301b1548 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -557,6 +557,14 @@ struct sched_entity { u64 prev_sum_exec_runtime; u64 nr_migrations; + /* + * The 'snapshot' of sum_exec_runtime when task + * voluntarily switches out. This is used to + * calculate the average duration below. + */ + u64 prev_sum_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 5800b0623ff3..f5281fe7f8f0 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4330,6 +4330,8 @@ 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_runtime_vol = 0; INIT_LIST_HEAD(&p->se.group_node); #ifdef CONFIG_FAIR_GROUP_SCHED diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e4a0b8bd941c..8820d0d14519 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6200,6 +6200,19 @@ static int wake_wide(struct task_struct *p) return 1; } +/* + * If a task switches in and then voluntarily relinquishes the + * CPU quickly, it is regarded as a short duration task. + * sysctl_sched_min_granularity is chosen as the threshold, + * as this value is the minimal slice if there are too many + * runnable tasks, see __sched_period(). + */ +static inline int is_short_task(struct task_struct *p) +{ + return sched_feat(SIS_SHORT) && + (p->se.dur_avg <= sysctl_sched_min_granularity); +} + /* * The purpose of wake_affine() is to quickly determine on which CPU we can run * soonest. For the purpose of speed we only consider the waking and previous @@ -7679,6 +7692,70 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev) { struct sched_entity *se = &prev->se; struct cfs_rq *cfs_rq; + u64 this_dur_avg, last_dur_avg; + long delta; + + /* + * Calculate the task's average duration. + * + * Only consider that task voluntarily relinquishes the CPU. + * For example, suppose on CPU1, task p1 and p2 runs + * alternatively: + * + * --------------------> time + * + * | p1 runs 1ms | p2 preempt p1 | p1 runs 0.5ms and sleeps | + * ^ ^ ^ ^ + * |_____________| |__________________________| + * |____________________________________| + * p1's duration + * + * + * The duration of p1 is 1.5ms rather than 0.5ms or 1ms + * in above case. That is to say, the duration period starts + * when task p1 switches in, ends when task p1 voluntarily + * relinquishes the CPU. This duration descibes the "nature" + * of a task: If a task is not preempted, how long it will + * run. + * + * The Exponential Weighted Moving Average (EWMA) + * is used to calculate the average duration. + * Borrowed from util_est_update(): + * + * ewma(t) = w * this_dur_avg + (1-w) * ewma(t-1) + * + * When 'w' is 0.125, it becomes update_avg(). + * This indicates that we care about approximately + * the recent 1 / 0.125 = 8 history duration. + */ + if (sched_feat(SIS_SHORT) && !prev->on_rq) { + /* + * sum_exec_runtime has been updated in update_curr() + * because we reach here via dequeue. + */ + this_dur_avg = se->sum_exec_runtime - se->prev_sum_runtime_vol; + /* + * Record the accumulated runtime when task voluntarily + * switches out. End of old duration period, a new period + * starts. + */ + se->prev_sum_runtime_vol = se->sum_exec_runtime; + + last_dur_avg = se->dur_avg; + delta = this_dur_avg - last_dur_avg; + /* consider large change to avoid frequent update */ + if (abs(delta) >= sysctl_sched_min_granularity) { + /* + * If it is the first time the task starts to + * record dur_avg, discard the initial value 0. + * Otherwise, calculate the EWMA. + */ + if (unlikely(!this_dur_avg)) + se->dur_avg = this_dur_avg; + else + update_avg(&se->dur_avg, this_dur_avg); + } + } for_each_sched_entity(se) { cfs_rq = cfs_rq_of(se); 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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