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

[Ankur Arora]

Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>> Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
>> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
>> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
>> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
>> ---
>>   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