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

Steven Rostedt
2023-10-18T14:30:15Z

On Wed, 18 Oct 2023 15:16:12 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> > 14. The kernel/trace/trace_osnoise.c file's run_osnoise() function
> > might need to do something for non-preemptible RCU to make
> > up for the lack of cond_resched() calls. Maybe just drop the
> > "IS_ENABLED()" and execute the body of the current "if" statement
> > unconditionally.

Right.

I'm guessing you are talking about this code:

/*
* In some cases, notably when running on a nohz_full CPU with
* a stopped tick PREEMPT_RCU has no way to account for QSs.
* This will eventually cause unwarranted noise as PREEMPT_RCU
* will force preemption as the means of ending the current
* grace period. We avoid this problem by calling
* rcu_momentary_dyntick_idle(), which performs a zero duration
* EQS allowing PREEMPT_RCU to end the current grace period.
* This call shouldn't be wrapped inside an RCU critical
* section.
*
* Note that in non PREEMPT_RCU kernels QSs are handled through
* cond_resched()
*/
if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
if (!disable_irq)
local_irq_disable();

rcu_momentary_dyntick_idle();

if (!disable_irq)
local_irq_enable();
}

/*
* For the non-preemptive kernel config: let threads runs, if
* they so wish, unless set not do to so.
*/
if (!disable_irq && !disable_preemption)
cond_resched();

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.

>
> Again. There is no non-preemtible RCU with this model, unless I'm
> missing something important here.

Daniel?

-- Steve