Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Tue, 21 Nov 2023 14:26:33 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        torvalds@linux-foundation.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,
        willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com,
        bharata@amd.com, raghavendra.kt@amd.com,
        boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
        jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org,
        bristot@kernel.org, mathieu.desnoyers@efficios.com,
        geert@linux-m68k.org, glaubitz@physik.fu-berlin.de,
        anton.ivanov@cambridgegreys.com, mattst88@gmail.com,
        krypton@ulrich-teichert.org, David.Laight@aculab.com,
        richard@nod.at, mjguzik@gmail.com
Subject: Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n
Message-ID: <4605b4f4-8a2b-4653-b684-9c696c36ebd0@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20231107215742.363031-1-ankur.a.arora@oracle.com>
 <20231107215742.363031-49-ankur.a.arora@oracle.com>
 <2027da00-273d-41cf-b9e7-460776181083@paulmck-laptop>
 <87lear4wj6.fsf@oracle.com>
 <46a4c47a-ba1c-4776-a6f8-6c2146cbdd0d@paulmck-laptop>
 <31d50051-e42c-4ef2-a1ac-e45370c3752e@paulmck-laptop>
 <d10b6243-41b1-44a0-ba95-0cedc7f6856e@paulmck-laptop>
 <20231121203049.GN8262@noisy.programming.kicks-ass.net>
 <1cdbb0f6-9078-4023-bf37-8d826ca0c711@paulmck-laptop>
 <20231121163834.571abb52@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231121163834.571abb52@gandalf.local.home>
Precedence: bulk

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@kernel.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