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 15:19:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, williams@...hat.com,
        daniel@...stot.me, 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 08:39:30AM -0400, Steven Rostedt wrote:
> I believe I see what Daniel is talking about, but I hate the proposed
> solution ;-)
> 
> First, if you care about real times that the CPU can't preempt
> (preempt_count != 0 or interrupts disabled), then you want the
> preempt_irqsoff tracer. The preempt_tracer is more academic where it
> just shows you when we disable preemption via the counter. But even
> with the preempt_irqsoff tracer you may not get the full length of time
> due to the above explained race.

IOW, that tracer gives a completely 'make believe' number? What's the
point? Just delete the pure preempt tracer.

And the preempt_irqoff tracer had better also consume the IRQ events,
and if it does that it can DTRT without extra bits on, even with that
race.

Consider:

	preempt_disable()
	  preempt_count += 1;
	  <IRQ>
	    trace_irq_enter();

	    trace_irq_exit();
	  </IRQ>
	  trace_preempt_disable();

	/* does stuff */

	preempt_enable()
	  preempt_count -= 1;
	  trace_preempt_enable();

You're saying preempt_irqoff() fails to connect the two because of the
hole between trace_irq_exit() and trace_preempt_disable() ?

But trace_irq_exit() can see the raised preempt_count and set state for
trace_preempt_disable() to connect.

> What I would recommend is adding a flag to the task_struct that gets
> set before the __preempt_count_add() and cleared by the tracing
> function. If an interrupt goes off during this time, it will start the
> total time to record, and not end it on the trace_hardirqs_on() part.
> Now since we set this flag before disabling preemption, what if we get
> preempted before calling __preempt_count_add()?. Simple, have a hook in
> the scheduler (just connect to the sched_switch tracepoint) that checks
> that flag, and if it is set, it ends the preempt disable recording
> time. Also on scheduling that task back in, if that flag is set, start
> the preempt disable timer.

I don't think that works, you also have to consider softirq. And yes you
can make it more complicated, but I still don't see the point.

And none of this is relevant for Daniels model stuff. He just needs to
consider in-IRQ as !preempt.