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

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Message-ID: <20231121175209.1d7ec202@gandalf.local.home>
Date:   Tue, 21 Nov 2023 17:52:09 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        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 Tue, 21 Nov 2023 14:26:33 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> On Tue, Nov 21, 2023 at 04:38:34PM -0500, Steven Rostedt wrote:
> > On Tue, 21 Nov 2023 13:14:16 -0800
> > "Paul E. McKenney" <paulmck@...nel.org> wrote:
> >   
> > > On Tue, Nov 21, 2023 at 09:30:49PM +0100, Peter Zijlstra wrote:  
> > > > On Tue, Nov 21, 2023 at 11:25:18AM -0800, Paul E. McKenney wrote:    
> > > > > #define preempt_enable() \
> > > > > do { \
> > > > > 	barrier(); \
> > > > > 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && raw_cpu_read(rcu_data.rcu_urgent_qs) && \
> > > > > 	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) &&
> > > > > 	    !irqs_disabled()) \  
> > 
> > Could we make the above an else case of the below if ?  
> 
> Wouldn't that cause the above preempt_count() test to always fail?

preempt_count_dec_and_test() returns true if preempt_count() is zero, which
happens only if NEED_RESCHED is set, and the rest of preempt_count() is not
set. (NEED_RESCHED bit in preempt_count() is really the inverse of
NEED_RESCHED). Do we need to call rcu_all_qs() when we call the scheduler?
Isn't scheduling a quiescent state for most RCU flavors?

I thought this was to help move along the quiescent states without added
cond_resched() around, which has:

int __sched __cond_resched(void)
{
	if (should_resched(0)) {
		preempt_schedule_common();
		return 1;
	}
	/*
	 * In preemptible kernels, ->rcu_read_lock_nesting tells the tick
	 * whether the current CPU is in an RCU read-side critical section,
	 * so the tick can report quiescent states even for CPUs looping
	 * in kernel context.  In contrast, in non-preemptible kernels,
	 * RCU readers leave no in-memory hints, which means that CPU-bound
	 * processes executing in kernel context might never report an
	 * RCU quiescent state.  Therefore, the following code causes
	 * cond_resched() to report a quiescent state, but only when RCU
	 * is in urgent need of one.
	 */
#ifndef CONFIG_PREEMPT_RCU
	rcu_all_qs();
#endif
	return 0;
}

Where if we schedule, we don't call rcu_all_qs().

I stand by that being in the else statement. It looks like that would keep
the previous work flow.

-- Steve