Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Daniel Bristot de Oliveira
2023-10-19T12:37:36Z

On 10/18/23 20:13, Paul E. McKenney wrote:
> On Wed, Oct 18, 2023 at 02:00:35PM -0400, Steven Rostedt wrote:
>> On Wed, 18 Oct 2023 10:55:02 -0700
>> "Paul E. McKenney" <paulmck@kernel.org> wrote:
>>
>>>> If everything becomes PREEMPT_RCU, then the above should be able to be
>>>> turned into just:
>>>>
>>>> if (!disable_irq)
>>>> local_irq_disable();
>>>>
>>>> rcu_momentary_dyntick_idle();
>>>>
>>>> if (!disable_irq)
>>>> local_irq_enable();
>>>>
>>>> And no cond_resched() is needed.
>>>
>>> Even given that CONFIG_PREEMPT_RCU=n still exists, the fact that
>>> run_osnoise() is running in kthread context with preemption and everything
>>> else enabled (am I right?), then the change you suggest should work fine.
>>
>> There's a user space option that lets you run that loop with preemption and/or
>> interrupts disabled.
>
> Ah, thank you. Then as long as this function is not expecting an RCU
> reader to span that call to rcu_momentary_dyntick_idle(), all is well.
> This is a kthread, so there cannot be something else expecting an RCU
> reader to span that call.

Sorry for the delay, this thread is quite long (and I admit I should be paying
attention to it).

It seems that you both figure it out without me anyways. This piece of
code is preemptive unless a config is set to disable irq or preemption (as
steven mentioned). That call is just a ping to RCU to say that things
are fine.

So Steven's suggestion should work.

>>>>> Again. There is no non-preemtible RCU with this model, unless I'm
>>>>> missing something important here.
>>>>
>>>> Daniel?
>>>
>>> But very happy to defer to Daniel. ;-)
>>
>> But Daniel could also correct me ;-)
>
> If he figures out a way that it is broken, he gets to fix it. ;-)

It works for me, keep in the loop for the patches and I can test and
adjust osnoise accordingly. osnoise should not be a reason to block more
important things like this patch set, and we can find a way out in
the osnoise tracer side. (I might need an assistance from rcu
people, but I know I can count on them :-).

Thanks!
-- Daniel
> Thanx, Paul