Messages in this thread |  | | From | Joel Fernandes <> | Subject | Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check | Date | Sat, 3 Dec 2022 10:35:46 -0500 |
| |
> 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.
Thanks.
> > thanks, > Chenyu
|  |