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Date:   Wed, 29 May 2019 15:29:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel@vger.kernel.org, williams@redhat.com,
        daniel@bristot.me, 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: <20190529132926.GV2650@hirez.programming.kicks-ass.net>
References: <cover.1559051152.git.bristot@redhat.com>
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On Wed, May 29, 2019 at 03:19:57PM +0200, Peter Zijlstra wrote:
> 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.

Alternatively, fix the preempt tracer by having it completely disregard
IRQs.