Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for this_cpu_read/write()

Steven Rostedt
2011-09-21T01:33:32Z

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