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




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