Messages in this thread | | | Date | Mon, 24 Oct 2022 18:10:22 +0800 | Subject | Re: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check | From | Abel Wu <> |
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LGTM, and please check inline comments.
On 10/23/22 11:32 PM, Chen Yu wrote: > 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 min_granularity is default to 750us, and can be tuned higher on machines running batch workloads. So I'm not sure it is appropriate to define 'short' by 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;
Would it better initialize to 'short' (min_granularity in this case)? So we can get rid of this check.
> + 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
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