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

Tianchen Ding
2024-06-24T02:35:55Z

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)

So in the special case about SCHED_NORMAL vs SCHED_IDLE, I suggest still do some
work in fair.c.

Thanks.