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Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()
From: Steven Rostedt <rostedt@goodmis.org>
To: Valdis.Kletnieks@vt.edu
Cc: Christoph Lameter <cl@gentwo.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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Date: Tue, 20 Sep 2011 21:33:27 -0400
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On Tue, 2011-09-20 at 18:17 -0400, Valdis.Kletnieks@vt.edu wrote:
> On Tue, 20 Sep 2011 11:19:47 EDT, Steven Rostedt said:
> > On Tue, 2011-09-20 at 09:57 -0500, Christoph Lameter wrote:
> > 
> > > this_cpu_xx functions are made for those locations that have
> > > preemption enabled. If you can use those function (classic case is a
> > > per cpu counter increment in the network subsystem) then you can avoid
> > > preempt disable/enable or get_cpu/put_cpu.
> > 
> > If the variables are used for a very short time, then the latencies
> > introduced by a simple:
> > 
> > 	var = get_cpu_var(my_var);
> > 	if (var)
> > 		do_something_quick();
> > 	put_cpu_var(my_var);
> > 
> > Otherwise if that do_something_quick(); migrates, it may be doing
> > something it shouldn't be doing!
> 
> This has the added advantage of making the calling function take the blame
> in latency traces, doesn't it?

Yes, the preempt off latency tracer would detect the above, if it took
too long.

-- Steve