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Date:   Fri, 27 Apr 2018 08:58:46 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Joel Fernandes <joelaf@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Tom Zanussi <tom.zanussi@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Boqun Feng <boqun.feng@gmail.com>,
        fweisbec <fweisbec@gmail.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        kbuild test robot <fengguang.wu@intel.com>,
        baohong liu <baohong.liu@intel.com>,
        vedang patel <vedang.patel@intel.com>, kernel-team@android.com
Subject: Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing
 with preempt on
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On Fri, Apr 27, 2018 at 11:40:05AM -0400, Steven Rostedt wrote:
> On Fri, 27 Apr 2018 08:38:26 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > On Fri, Apr 27, 2018 at 10:47:47AM -0400, Steven Rostedt wrote:
> > > On Fri, 27 Apr 2018 10:26:29 -0400 (EDT)
> > > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> > >   
> > > > The general approach and the implementation look fine, except for
> > > > one small detail: I would be tempted to explicitly disable preemption
> > > > around the call to the tracepoint callback for the rcuidle variant,
> > > > unless we plan to audit every tracer right away to remove any assumption
> > > > that preemption is disabled in the callback implementation.  
> > > 
> > > I'm thinking that we do that audit. There shouldn't be many instances
> > > of it. I like the idea that a tracepoint callback gets called with
> > > preemption enabled.  
> > 
> > Are you really sure you want to increase your state space that much?
> 
> Why not? The code I have in callbacks already deals with all sorts of
> context - normal, softirq, irq, NMI, preemption disabled, irq
> disabled.

But why?  Do people really expect good real-time response on systems
instrumented with lots of tracing?

								Thanx, Paul