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Date:   Fri, 31 May 2019 03:47:29 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>,
        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: <20190531074729.GA153831@google.com>
References: <cover.1559051152.git.bristot@redhat.com>
 <f2ca7336162b6dc45f413cfe4e0056e6aa32e7ed.1559051152.git.bristot@redhat.com>
 <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.
> 
> But there are other use-cases, for instance:
> 
> (Steve, correct me if I am wrong)
> 
> The preempt_tracer will not notice a "preempt disabled" section in an IRQ
> handler if the problem above happens.
> 
> (Yeah, I know these problems are very specific... but...)

I agree with the problem. I think Daniel does not want to miss the preemption
disabled event caused by the IRQ disabling.

> >> To avoid skipping the trace, the change in the counter should be "atomic"
> >> with the start/stop, w.r.t the interrupts.
> >>
> >> Disable interrupts while the adding/starting stopping/subtracting.
> > 
> >> +static inline void preempt_add_start_latency(int val)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	raw_local_irq_save(flags);
> >> +	__preempt_count_add(val);
> >> +	preempt_latency_start(val);
> >> +	raw_local_irq_restore(flags);
> >> +}
> > 
> >> +static inline void preempt_sub_stop_latency(int val)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	raw_local_irq_save(flags);
> >> +	preempt_latency_stop(val);
> >> +	__preempt_count_sub(val);
> >> +	raw_local_irq_restore(flags);
> >> +}
> > 
> > That is hideously expensive :/
> 
> Yeah... :-( Is there another way to provide such "atomicity"?
> 
> Can I use the argument "if one has these tracepoints enabled, they are not
> considering it as a hot-path?"

The only addition here seems to  the raw_local_irq_{save,restore} around the
calls to increment the preempt counter and start the latency tracking.

Is there any performance data with the tracepoint enabled and with/without
this patch? Like with hackbench?

Thanks.