Re: [PATCH v2 29/35] sched: handle preempt=voluntary under PREEMPT_AUTO

[Ankur Arora <ankur.a.arora@oracle.com>]

Tianchen Ding <dtcccc@linux.alibaba.com> writes:

> On 2024/6/22 02:58, Ankur Arora wrote:
>> Tianchen Ding <dtcccc@linux.alibaba.com> writes:
>>
>>> On 2024/5/28 08:35, Ankur Arora wrote:
>>>> The default preemption policy for voluntary preemption under
>>>> PREEMPT_AUTO is to schedule eagerly for tasks of higher scheduling
>>>> class, and lazily for well-behaved, non-idle tasks.
>>>> This is the same policy as preempt=none, with an eager handling of
>>>> higher priority scheduling classes.
>>>> Comparing a cyclictest workload with a background kernel load of
>>>> 'stress-ng --mmap', shows that both the average and the maximum
>>>> latencies improve:
>>>>    # stress-ng --mmap 0 &
>>>>    # cyclictest --mlockall --smp --priority=80 --interval=200 --distance=0 -q -D 300
>>>>                                        Min     (  %stdev )    Act     (  %stdev
>>>> )   Avg     (  %stdev )   Max      (  %stdev )
>>>>     PREEMPT_AUTO, preempt=voluntary    1.73  ( +-  25.43% )   62.16 ( +-
>>>> 303.39% )  14.92 ( +-  17.96% )  2778.22 ( +-  15.04% )
>>>>     PREEMPT_DYNAMIC, preempt=voluntary 1.83  ( +-  20.76% )  253.45 ( +- 233.21% )  18.70 ( +-  15.88% )  2992.45 ( +-  15.95% )
>>>> The table above shows the aggregated latencies across all CPUs.
>>>> Cc: Ingo Molnar <mingo@redhat.com>
>>>> Cc: Peter Ziljstra <peterz@infradead.org>
>>>> Cc: Juri Lelli <juri.lelli@redhat.com>
>>>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>>>> Originally-by: Thomas Gleixner <tglx@linutronix.de>
>>>> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
>>>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>>>> ---
>>>>    kernel/sched/core.c  | 12 ++++++++----
>>>>    kernel/sched/sched.h |  6 ++++++
>>>>    2 files changed, 14 insertions(+), 4 deletions(-)
>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>>> index c25cccc09b65..2bc3ae21a9d0 100644
>>>> --- a/kernel/sched/core.c
>>>> +++ b/kernel/sched/core.c
>>>> @@ -1052,6 +1052,9 @@ static resched_t resched_opt_translate(struct task_struct *curr,
>>>>    	if (preempt_model_preemptible())
>>>>    		return RESCHED_NOW;
>>>>    +	if (preempt_model_voluntary() && opt == RESCHED_PRIORITY)
>>>> +		return RESCHED_NOW;
>>>> +
>>>>    	if (is_idle_task(curr))
>>>>    		return RESCHED_NOW;
>>>>    @@ -2289,7 +2292,7 @@ void wakeup_preempt(struct rq *rq, struct task_struct
>>>> *p, int flags)
>>>>    	if (p->sched_class == rq->curr->sched_class)
>>>>    		rq->curr->sched_class->wakeup_preempt(rq, p, flags);
>>>>    	else if (sched_class_above(p->sched_class, rq->curr->sched_class))
>>>> -		resched_curr(rq);
>>>> +		resched_curr_priority(rq);
>>>>
>>> Besides the conditions about higher class, can we do resched_curr_priority() in the same class?
>>> For example, in fair class, we can do it when SCHED_NORMAL vs SCHED_IDLE.
>> So, I agree about the specific case of SCHED_NORMAL vs SCHED_IDLE.
>> (And, that case is already handled by resched_opt_translate() explicitly
>> promoting idle tasks to TIF_NEED_RESCHED.)
>> But, on the general question of doing resched_curr_priority() in the
>> same class: I did consider it. But, it seemed to me that we want to
>> keep run to completion semantics for lazy scheduling, and so not
>> enforcing priority in a scheduling class was a good line.
>>
>
> OK, on general question, this is just a suggestion :-)
>
> Actually, my key point is about SCHED_IDLE. It's not a real idle task, but a
> normal task with lowest priority. So is_idle_task() in resched_opt_translate()
> does not fit it. Should add task_has_idle_policy().
>
> However, even using task_has_idle_policy() may be still not enough. Because
> SCHED_IDLE policy:
>   1. It is the lowest priority, but still belongs to fair_sched_class, which is
>   the same as SCHED_NORMAL.
>   2. Not only tasks, *se of cgroup* can be SCHED_IDLE, too. (introduced by
>   commit 304000390f88d)

Thanks. That is useful to know. Let me see how best to incorporate this.

Side question: are there any benchmarks that would exercise various types
of sched policy, idle and otherwise?

--
ankur