Messages in this thread | | | Date | Tue, 20 Sep 2022 15:18:16 +0200 | Subject | Re: [PATCH v4 5/8] sched/fair: Take into account latency priority at wakeup | From | Dietmar Eggemann <> |
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On 19/09/2022 17:39, Vincent Guittot wrote: > On Mon, 19 Sept 2022 at 12:05, Dietmar Eggemann > <dietmar.eggemann@arm.com> wrote: >> >> On 16/09/2022 10:03, Vincent Guittot wrote: >> >> [...] >> >>> @@ -4512,7 +4519,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p) >>> p->prio = current->normal_prio; >>> >>> /* Propagate the parent's latency requirements to the child as well */ >>> - p->latency_nice = current->latency_nice; >>> + p->latency_prio = current->latency_prio; >> >> Isn't here a `set_latency_offset(p)` missing here? > > Hmm, I think it's the opposite and the line above is a nop from the > beginning (i.e. patch 2).
Yeah, you're right! It looked suspicious ...
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>>> + * the idle thread and don't set next buddy as a candidate for being >>> + * picked in priority. >>> + * In case of simultaneous wakeup from idle, the latency sensitive tasks >>> + * lost opportunity to preempt non sensitive tasks which woke up >>> + * simultaneously. >>> + */ >> >> The position of this comment block within this function is somehow >> misleading since it describes the reason for the function rather then a >> particular condition within this function. Wouldn't it be more readable >> when it would be a function header comment instead? > > I put it after the usual early return tests to put the comment close > to the useful part: the use of next buddy and __pick_first_entity()
So you want to have the `wakeup_preempt_entity(se, pse) == 1` condition from check_preempt_wakeup() also for cfs_task woken up by others.
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>>> + * requirement that needs to be evaluated versus other entity. >>> + * Otherwise, use the latency weight to evaluate how much scheduling >>> + * delay is acceptable by se. >>> + */ >>> + if ((se->latency_offset < 0) || (curr->latency_offset < 0)) >>> + latency_offset -= curr->latency_offset; >> >> I still don't get the rationale behind why when either one (se or curr) >> of the latency_nice values is negative, we use the diff between them but >> if not, we only care about se's value. Why don't you always use the diff >> between se and curr? Since we have a range [-20 ... 19] why shouldn't we >> use the difference between let's say se = 19 and curr = 5? >> You discussed this with Tao Zhou on the v1 but I didn't understand it fully. > > Let say that current has a latency nice prio of 19 and a task A with a > latency nice of 10 wakes up. Both tasks don't care about scheduling > latency (current more than task A). If we use the diff, the output of > wakeup_latency_gran() would be negative (-10ms) which reflects the > fact that the waking task is sensitive to the latency and wants to > preempt current even if its vruntime is after. But obviously both > current and task A don't care to preempt at wakeup.
OK, I understand but there is a certain level of unsteadiness here.
If p has >0 it gets treated differently in case current has >=0 and case current has <0.
Do we expect that tasks set their value to [1..19] in this case, when the default 0 already indicates a 'don't care'?
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