linux-kernel - Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt

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Date:   Wed, 29 May 2019 12:20:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org, williams@...hat.com,
        daniel@...stot.me, "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yangtao Li <tiny.windzz@...il.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping
 due to a preempt_counter change

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.