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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
Date: Wed, 29 May 2019 12:20:38 +0200	[thread overview]
Message-ID: <20190529102038.GO2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <b47631c3-d65a-4506-098a-355c8cf50601@redhat.com>

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.


  reply	other threads:[~2019-05-29 10:21 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-28 15:16 [RFC 0/3] preempt_tracer: Fix preempt_disable tracepoint Daniel Bristot de Oliveira
2019-05-28 15:16 ` [RFC 1/3] softirq: Use preempt_latency_stop/start to trace preemption Daniel Bristot de Oliveira
2019-05-29  8:29   ` Peter Zijlstra
2019-05-29  9:30   ` Joel Fernandes
2019-05-29 12:22     ` Steven Rostedt
2019-06-04 10:39       ` Daniel Bristot de Oliveira
2019-06-04 12:01         ` Steven Rostedt
2019-05-28 15:16 ` [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change Daniel Bristot de Oliveira
2019-05-29  8:33   ` Peter Zijlstra
2019-05-29  9:40     ` Daniel Bristot de Oliveira
2019-05-29 10:20       ` Peter Zijlstra [this message]
2019-05-29 12:39         ` Steven Rostedt
2019-05-29 13:19           ` Peter Zijlstra
2019-05-29 13:29             ` Peter Zijlstra
2019-05-29 13:42             ` Steven Rostedt
2019-05-29 13:49               ` Peter Zijlstra
2019-05-29 13:58                 ` Steven Rostedt
2019-05-29 13:51         ` Daniel Bristot de Oliveira
2019-05-29 18:21           ` Peter Zijlstra
2019-06-04 10:20             ` Daniel Bristot de Oliveira
2019-05-31  7:47       ` Joel Fernandes
2019-06-04 10:12         ` Daniel Bristot de Oliveira
2019-06-04 15:01           ` Steven Rostedt
2019-06-05 15:16             ` Joel Fernandes
2019-05-28 15:16 ` [RFC 3/3] preempt_tracer: Use a percpu variable to control traceble calls Daniel Bristot de Oliveira
2019-05-29  8:41   ` Peter Zijlstra
2019-05-29  9:48     ` Daniel Bristot de Oliveira

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