Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 5 Dec 2022 10:59:06 +0100 | Subject | Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check |
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On Mon, 5 Dec 2022 at 09:39, Chen Yu <yu.c.chen@intel.com> wrote: > > Hi Joel, > On 2022-12-03 at 10:35:46 -0500, Joel Fernandes wrote: > > > > > > > On Dec 3, 2022, at 2:50 AM, Chen Yu <yu.c.chen@intel.com> wrote: > > > > > > 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. > > > > There are already ways to misconfigure sched sysctl to make bad/weird things happen. > > > > >> 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. > > > > It would be nice to not have to introduce a new knob for this. IMO, min_granularity is reasonable. > > > OK, got it, thanks for the suggestion.
Sorry for the late answer. We don't want to add more dedicated knobs. So using sysctl_sched_min_granularity as you are doing in this patch looks ok
> > thanks, > Chenyu
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