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Date:   Wed, 18 Oct 2023 11:13:17 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
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        bharata@amd.com, raghavendra.kt@amd.com,
        boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
        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, 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.

> > > > 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.  ;-)

						Thanx, Paul