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

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Message-ID: <4605b4f4-8a2b-4653-b684-9c696c36ebd0@paulmck-laptop>
Date:   Tue, 21 Nov 2023 14:26:33 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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, 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?

Another approach is to bury the test in preempt_count_dec_and_test(),
but I suspect that this would not make Peter any more happy than my
earlier suggestion.  ;-)

> > > > 		rcu_all_qs(); \
> > > > 	if (unlikely(preempt_count_dec_and_test())) { \
> > > > 		__preempt_schedule(); \
> > > > 	} \
> > > > } while (0)  
> > > 
> > > Aaaaahhh, please no. We spend so much time reducing preempt_enable() to
> > > the minimal thing it is today, this will make it blow up into something
> > > giant again.  
> 
> Note, the above is only true with "CONFIG_PREEMPT_RCU is not set", which
> keeps the preempt_count() for preemptable kernels with PREEMPT_RCU still minimal.

Agreed, and there is probably some workload that does not like this.
After all, current CONFIG_PREEMPT_DYNAMIC=y booted with preempt=none
would have those cond_resched() invocations.  I was leary of checking
dynamic information, but maybe sched_feat() is faster than I am thinking?
(It should be with the static_branch, but not sure about the other two
access modes.)

							Thanx, Paul