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Date:   Wed, 18 Oct 2023 10:31:46 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     paulmck@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
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        jgross@suse.com, andrew.cooper3@citrix.com,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Daniel Bristot de Oliveira <bristot@kernel.org>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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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