linux-kernel - Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT

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Message-ID: <3cc64b80-0975-4cdf-81ea-952ca80976e4@paulmck-laptop>
Date:   Wed, 6 Dec 2023 20:37:07 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        torvalds@...ux-foundation.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, David.Laight@...lab.com,
        richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

On Wed, Dec 06, 2023 at 09:10:22PM -0500, Steven Rostedt wrote:
> On Wed, 06 Dec 2023 17:31:30 -0800
> Ankur Arora <ankur.a.arora@...cle.com> wrote:
> 
> > ---
> > diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> > index 9aa6358a1a16..d8139cda8814 100644
> > --- a/include/linux/preempt.h
> > +++ b/include/linux/preempt.h
> > @@ -226,9 +226,11 @@ do { \
> >  #ifdef CONFIG_PREEMPTION
> >  #define preempt_enable() \
> >  do { \
> >  	barrier(); \
> >  	if (unlikely(preempt_count_dec_and_test())) \
> >  		__preempt_schedule(); \
> > +	else if (unlikely(raw_cpu_read(rcu_data.rcu_urgent_qs))) \
> 
> Shouldn't this still have the:
> 
> 	else if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && \
> 
> That is, is it needed when PREEMPT_RCU is set?

Given that PREEMPT_RCU has been getting along fine without it, I agree
with Steve on this one.  Unless and until someone demonstrates otherwise,
but such a demonstration would almost certainly affect current code,
not just the lazy-preemption changes.

							Thanx, Paul

> -- Steve
> 
> 
> > +		rcu_all_qs_check();
> >  } while (0)
> > 
> >  #define preempt_enable_notrace() \