Messages in this thread | | | Date | Thu, 22 Sep 2022 00:41:40 +0200 | Subject | Re: [PATCH v4 5/8] sched/fair: Take into account latency priority at wakeup | From | Dietmar Eggemann <> |
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On 20/09/2022 17:49, Vincent Guittot wrote: > On Tue, 20 Sept 2022 at 15:18, Dietmar Eggemann > <dietmar.eggemann@arm.com> wrote: >> >> 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:
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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. > > I wake the wakeup_preempt_entity(cfs_rq->next, left) < 1 in > pick_next_entity() to pick the task with highest latency constraint > when another class is running while waking up
That's correct. This is where you potentially pick this task since it is the next_buddy. All I wanted to say is that check_preempt_from_others() and its `next && wakeup_preempt_entity(next, se) == 1` is the counterpart of the `wakeup_preempt_entity(se, pse) == 1` in check_preempt_wakeup() to be able to set next_buddy even curr is from an other class than CFS.
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>>>> 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 > > "If p >=0"; 0 has same behavior than [1..19] > >> current has <0.
Not quite. It depends on curr. With sysctl_sched_latency = 24ms:
(1) p = 10 curr = 19 -> wakeup_latency_gran() returns 12ms
(2) p = 10 curr = -10 -> wakeup_latency_gran() returns 24ms
In (1) only p's own latency counts whereas in (2) we take the diff,
In (A) we 'punish' p even though it competes against curr which has an even lower latency requirement than p,
>> Do we expect that tasks set their value to [1..19] in this case, when >> the default 0 already indicates a 'don't care'? > > I'm not sure that I understand your concern as [0..19] are treated in > the same way. Only tasks (curr or se) with offset <0 need a relative > comparison to the other. If curr and se has both a latency nice of > -19, se should not blindly preempt curr but only if curr already run > for its amount of time
With p = -19 and curr = -19 we would take the diff, so 0ms.
With p = 19 and curr = 19, if we would use `latency_offset -= curr->latency_offset` wakeup_latency_gran() would return 973/1024*24ms - 973/1024*24ms = 0ms and nothing will shift.
OTHA, in case (1) wakeup_latency_gran() would return 512/1024*24ms - 973/1024*24ms = - 10.80ms. So p would gain an advantage here instead of a penalty.
Essentially using the full [-20 .. 19] nice scope for `se vs. curr` comparison.
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