Messages in this thread |  | | Date | Sat, 3 Dec 2022 15:49:30 +0800 | From | Chen Yu <> | Subject | Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check |
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Hi Honglei, On 2022-12-02 at 15:44:18 +0800, Honglei Wang wrote: > > > On 2022/12/1 16:44, Chen Yu wrote: > > Introduce short-duration task checks, as there is 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. > > > > Although something based on PELT could 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 6ms and sleeps for 32ms, p3 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. Introduce SIS_SHORT to control this strategy. > > > > The threshold of short duration reuses sysctl_sched_min_granularity, > > so it can be tuned by the user. Ideally there should be a dedicated > > parameter for the threshold, but that might introduce complexity. > > > > 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 | 4 ++++ > > kernel/sched/core.c | 2 ++ > > kernel/sched/fair.c | 17 +++++++++++++++++ > > kernel/sched/features.h | 1 + > > 4 files changed, 24 insertions(+) > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index ffb6eb55cd13..64b7acb77a11 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -558,6 +558,10 @@ struct sched_entity { > > u64 nr_migrations; > > + u64 prev_sum_exec_runtime_vol; > > + /* average duration of a task */ > > + u64 dur_avg; > > + > > #ifdef CONFIG_FAIR_GROUP_SCHED > > int depth; > > struct sched_entity *parent; > > 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/fair.c b/kernel/sched/fair.c > > index e4a0b8bd941c..a4b314b664f8 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6200,6 +6200,16 @@ 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. > > + */ > > +static inline int is_short_task(struct task_struct *p) > > +{ > > + return sched_feat(SIS_SHORT) && > > + (p->se.dur_avg <= sysctl_sched_min_granularity); > > +} > > + > > Hi Yu, > > I still have a bit concern about the sysctl_sched_min_granularity stuff.. > This grab can be set to different value which will impact the action of this > patch and make things not totally under control. > > Not sure if we can add a new grab for this.. The test result shows good > improvement for short task, and with this grab, admins will be able to > custom the system base on their own 'short task' view. > It would be ideal to have a dedicated parameter to tweak this. For example, something under /sys/kernel/debug/sched/, and initilized to sysctl_sched_min_granularity by default. Hi Peter, Vincent, may I have your opinion if this is applicable?
thanks, Chenyu
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