linux-kernel - Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for this_cpu

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Message-ID: <1316568807.29966.122.camel@gandalf.stny.rr.com>
Date:	Tue, 20 Sep 2011 21:33:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Valdis.Kletnieks@...edu
Cc:	Christoph Lameter <cl@...two.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()

On Tue, 2011-09-20 at 18:17 -0400, Valdis.Kletnieks@...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


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