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

Ankur Arora
2024-06-21T18:59:40Z

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.

(Note that resched_curr_priority(), at least as it stands, is going away
for v3. I'll be folding lazy scheduling as a single model under
PREEMPT_DYNAMIC. So, no separate lazy=none, lazy=voluntary.)

Thanks

--
ankur