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Date:   Wed, 29 May 2019 12:20:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     linux-kernel@vger.kernel.org, williams@redhat.com,
        daniel@bristot.me, "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Yangtao Li <tiny.windzz@gmail.com>,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Subject: Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping
 due to a preempt_counter change
Message-ID: <20190529102038.GO2623@hirez.programming.kicks-ass.net>
References: <cover.1559051152.git.bristot@redhat.com>
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 <20190529083357.GF2623@hirez.programming.kicks-ass.net>
 <b47631c3-d65a-4506-098a-355c8cf50601@redhat.com>
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On Wed, May 29, 2019 at 11:40:34AM +0200, Daniel Bristot de Oliveira wrote:
> On 29/05/2019 10:33, Peter Zijlstra wrote:
> > On Tue, May 28, 2019 at 05:16:23PM +0200, Daniel Bristot de Oliveira wrote:
> >> The preempt_disable/enable tracepoint only traces in the disable <-> enable
> >> case, which is correct. But think about this case:
> >>
> >> ---------------------------- %< ------------------------------
> >> 	THREAD					IRQ
> >> 	   |					 |
> >> preempt_disable() {
> >>     __preempt_count_add(1)
> >> 	------->	    smp_apic_timer_interrupt() {
> >> 				preempt_disable()
> >> 				    do not trace (preempt count >= 1)
> >> 				    ....
> >> 				preempt_enable()
> >> 				    do not trace (preempt count >= 1)
> >> 			    }
> >>     trace_preempt_disable();
> >> }
> >> ---------------------------- >% ------------------------------
> >>
> >> The tracepoint will be skipped.
> > 
> > .... for the IRQ. But IRQs are not preemptible anyway, so what the
> > problem?
> 
> 
> right, they are.
> 
> exposing my problem in a more specific way:
> 
> To show in a model that an event always takes place with preemption disabled,
> but not necessarily with IRQs disabled, it is worth having the preemption
> disable events separated from IRQ disable ones.
> 
> The main reason is that, although IRQs disabled postpone the execution of the
> scheduler, it is more pessimistic, as it also delays IRQs. So the more precise
> the model is, the less pessimistic the analysis will be.

I'm not sure I follow, IRQs disabled fully implies !preemptible. I don't
see how the model would be more pessimistic than reality if it were to
use this knowledge.

Any !0 preempt_count(), which very much includes (Hard)IRQ and SoftIRQ
counts, means non-preemptible.