Re: [PATCH 21/24] sched/eevdf: Allow shorter slices to wakeup-preempt

Chunxin Zang
2024-08-13T10:44:51Z

> On Aug 8, 2024, at 01:54, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Aug 05, 2024 at 08:24:24PM +0800, Chunxin Zang wrote:
>>> On Jul 27, 2024, at 18:27, Peter Zijlstra <peterz@infradead.org> wrote:
>
>>> +static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>> +{
>>> + if (!sched_feat(PREEMPT_SHORT))
>>> + return false;
>>> +
>>> + if (curr->vlag == curr->deadline)
>>> + return false;
>>> +
>>> + return !entity_eligible(cfs_rq, curr);
>>> +}
>
>> Can this be made more aggressive here? Something like , in the PREEMPT_SHORT
>> + NO_RUN_TO_PARITY combination, it could break the first deadline of the current
>> task. This can achieve better latency benefits in certain embedded scenarios, such as
>> high-priority periodic tasks.
>
> You are aware we have SCHED_DEADLINE for those, right?
>
> Why can't you use that? and what can we do to fix that.
>
>> +static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>> +{
>> + if (!sched_feat(PREEMPT_SHORT))
>> + return false;
>> +
>> + if (sched_feat(RUN_TO_PARITY) && curr->vlag == curr->deadline)
>> + return false;
>> +
>> + return !entity_eligible(cfs_rq, curr);
>> +}
>
> No, this will destroy the steady state schedule (where no tasks
> join/leave) and make it so that all tasks hug the lag=0 state
> arbitrarily close -- as allowed by the scheduling quanta.
>
> Yes, it will get you better latency, because nobody gets to actually run
> it's requested slice.
>
> The goal really is for tasks to get their request -- and yes that means
> you get to wait. PREEMPT_SHORT is already an exception to this rule, and
> it is very specifically limited to wake-ups so as to retain as much of
> the intended behaviour as possible.

I think I understand your point now. That approach does indeed seem more reasonable.
Next, I will try using 'request slice' to conduct more tests in my scenario.

>
>> Additionally, if possible, could you please include my name in this patch? I spent over a
>> month finding this solution and conducting the tests, and I hope to leave some trace of
>> my efforts during that time. This is also one of the reasons why I love Linux and am eager
>> to contribute to open source. I would be extremely grateful.
>
> I've made it the below. Does that work for you?

Heartfelt thanks :)

Chunxin

>
> ---
> Subject: sched/eevdf: Allow shorter slices to wakeup-preempt
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue Sep 26 14:32:32 CEST 2023
>
> Part of the reason to have shorter slices is to improve
> responsiveness. Allow shorter slices to preempt longer slices on
> wakeup.
>
> Task | Runtime ms | Switches | Avg delay ms | Max delay ms | Sum delay ms |
>
> 100ms massive_intr 500us cyclictest NO_PREEMPT_SHORT
>
> 1 massive_intr:(5) | 846018.956 ms | 779188 | avg: 0.273 ms | max: 58.337 ms | sum:212545.245 ms |
> 2 massive_intr:(5) | 853450.693 ms | 792269 | avg: 0.275 ms | max: 71.193 ms | sum:218263.588 ms |
> 3 massive_intr:(5) | 843888.920 ms | 771456 | avg: 0.277 ms | max: 92.405 ms | sum:213353.221 ms |
> 1 chromium-browse:(8) | 53015.889 ms | 131766 | avg: 0.463 ms | max: 36.341 ms | sum:60959.230 ms |
> 2 chromium-browse:(8) | 53864.088 ms | 136962 | avg: 0.480 ms | max: 27.091 ms | sum:65687.681 ms |
> 3 chromium-browse:(9) | 53637.904 ms | 132637 | avg: 0.481 ms | max: 24.756 ms | sum:63781.673 ms |
> 1 cyclictest:(5) | 12615.604 ms | 639689 | avg: 0.471 ms | max: 32.272 ms | sum:301351.094 ms |
> 2 cyclictest:(5) | 12511.583 ms | 642578 | avg: 0.448 ms | max: 44.243 ms | sum:287632.830 ms |
> 3 cyclictest:(5) | 12545.867 ms | 635953 | avg: 0.475 ms | max: 25.530 ms | sum:302374.658 ms |
>
> 100ms massive_intr 500us cyclictest PREEMPT_SHORT
>
> 1 massive_intr:(5) | 839843.919 ms | 837384 | avg: 0.264 ms | max: 74.366 ms | sum:221476.885 ms |
> 2 massive_intr:(5) | 852449.913 ms | 845086 | avg: 0.252 ms | max: 68.162 ms | sum:212595.968 ms |
> 3 massive_intr:(5) | 839180.725 ms | 836883 | avg: 0.266 ms | max: 69.742 ms | sum:222812.038 ms |
> 1 chromium-browse:(11) | 54591.481 ms | 138388 | avg: 0.458 ms | max: 35.427 ms | sum:63401.508 ms |
> 2 chromium-browse:(8) | 52034.541 ms | 132276 | avg: 0.436 ms | max: 31.826 ms | sum:57732.958 ms |
> 3 chromium-browse:(8) | 55231.771 ms | 141892 | avg: 0.469 ms | max: 27.607 ms | sum:66538.697 ms |
> 1 cyclictest:(5) | 13156.391 ms | 667412 | avg: 0.373 ms | max: 38.247 ms | sum:249174.502 ms |
> 2 cyclictest:(5) | 12688.939 ms | 665144 | avg: 0.374 ms | max: 33.548 ms | sum:248509.392 ms |
> 3 cyclictest:(5) | 13475.623 ms | 669110 | avg: 0.370 ms | max: 37.819 ms | sum:247673.390 ms |
>
> As per the numbers the, this makes cyclictest (short slice) it's
> max-delay more consistent and consistency drops the sum-delay. The
> trade-off is that the massive_intr (long slice) gets more context
> switches and a slight increase in sum-delay.
>
> Chunxin contributed did_preempt_short() where a task that lost slice
> protection from PREEMPT_SHORT gets rescheduled once it becomes
> in-eligible.
>
> [mike: numbers]
> Co-Developed-by: Chunxin Zang <zangchunxin@lixiang.com>
> Signed-off-by: Chunxin Zang <zangchunxin@lixiang.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
> Link: https://lkml.kernel.org/r/20240727105030.735459544@infradead.org
> ---
> kernel/sched/fair.c | 64 ++++++++++++++++++++++++++++++++++++++++++------
> kernel/sched/features.h | 5 +++
> 2 files changed, 61 insertions(+), 8 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -973,10 +973,10 @@ static void clear_buddies(struct cfs_rq
> * XXX: strictly: vd_i += N*r_i/w_i such that: vd_i > ve_i
> * this is probably good enough.
> */
> -static void update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> if ((s64)(se->vruntime - se->deadline) < 0)
> - return;
> + return false;
>
> /*
> * For EEVDF the virtual time slope is determined by w_i (iow.
> @@ -993,10 +993,7 @@ static void update_deadline(struct cfs_r
> /*
> * The task has consumed its request, reschedule.
> */
> - if (cfs_rq->nr_running > 1) {
> - resched_curr(rq_of(cfs_rq));
> - clear_buddies(cfs_rq, se);
> - }
> + return true;
> }
>
> #include "pelt.h"
> @@ -1134,6 +1131,38 @@ static inline void update_curr_task(stru
> dl_server_update(p->dl_server, delta_exec);
> }
>
> +static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> +{
> + if (!sched_feat(PREEMPT_SHORT))
> + return false;
> +
> + if (curr->vlag == curr->deadline)
> + return false;
> +
> + return !entity_eligible(cfs_rq, curr);
> +}
> +
> +static inline bool do_preempt_short(struct cfs_rq *cfs_rq,
> + struct sched_entity *pse, struct sched_entity *se)
> +{
> + if (!sched_feat(PREEMPT_SHORT))
> + return false;
> +
> + if (pse->slice >= se->slice)
> + return false;
> +
> + if (!entity_eligible(cfs_rq, pse))
> + return false;
> +
> + if (entity_before(pse, se))
> + return true;
> +
> + if (!entity_eligible(cfs_rq, se))
> + return true;
> +
> + return false;
> +}
> +
> /*
> * Used by other classes to account runtime.
> */
> @@ -1157,6 +1186,7 @@ static void update_curr(struct cfs_rq *c
> struct sched_entity *curr = cfs_rq->curr;
> struct rq *rq = rq_of(cfs_rq);
> s64 delta_exec;
> + bool resched;
>
> if (unlikely(!curr))
> return;
> @@ -1166,7 +1196,7 @@ static void update_curr(struct cfs_rq *c
> return;
>
> curr->vruntime += calc_delta_fair(delta_exec, curr);
> - update_deadline(cfs_rq, curr);
> + resched = update_deadline(cfs_rq, curr);
> update_min_vruntime(cfs_rq);
>
> if (entity_is_task(curr)) {
> @@ -1184,6 +1214,14 @@ static void update_curr(struct cfs_rq *c
> }
>
> account_cfs_rq_runtime(cfs_rq, delta_exec);
> +
> + if (rq->nr_running == 1)
> + return;
> +
> + if (resched || did_preempt_short(cfs_rq, curr)) {
> + resched_curr(rq);
> + clear_buddies(cfs_rq, curr);
> + }
> }
>
> static void update_curr_fair(struct rq *rq)
> @@ -8611,7 +8649,17 @@ static void check_preempt_wakeup_fair(st
> cfs_rq = cfs_rq_of(se);
> update_curr(cfs_rq);
> /*
> - * XXX pick_eevdf(cfs_rq) != se ?
> + * If @p has a shorter slice than current and @p is eligible, override
> + * current's slice protection in order to allow preemption.
> + *
> + * Note that even if @p does not turn out to be the most eligible
> + * task at this moment, current's slice protection will be lost.
> + */
> + if (do_preempt_short(cfs_rq, pse, se) && se->vlag == se->deadline)
> + se->vlag = se->deadline + 1;
> +
> + /*
> + * If @p has become the most eligible task, force preemption.
> */
> if (pick_eevdf(cfs_rq) == pse)
> goto preempt;
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -18,6 +18,11 @@ SCHED_FEAT(PLACE_REL_DEADLINE, true)
> * 0-lag point or until is has exhausted it's slice.
> */
> SCHED_FEAT(RUN_TO_PARITY, true)
> +/*
> + * Allow wakeup of tasks with a shorter slice to cancel RESPECT_SLICE for
> + * current.
> + */
> +SCHED_FEAT(PREEMPT_SHORT, true)
>
> /*
> * Prefer to schedule the task we woke last (assuming it failed