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    SubjectRe: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop

    * Frédéric Weisbecker <fweisbec@gmail.com> wrote:

    > 2008/12/18 Ingo Molnar <mingo@elte.hu>:
    > >
    > > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
    > >
    > >> Impact: fix a system hang on slow systems
    > >>
    > >> While testing the function graph tracer on VirtualBox, I had a system hang
    > >> immediatly after enabling the tracer.
    > >>
    > >> If hrtimer is enabled on kernel, a slow system can spend too much time
    > >> during tracing the hrtimer_interrupt which will do eternal loops,
    > >> assuming it always have to retry its process because too much time
    > >> elapsed during its time update. Now we provide a feature which lurks at
    > >> the number of retries on hrtimer_interrupt. After 10 retries, the
    > >> function graph tracer will definetly stop its tracing.
    > >
    > > hm, i dont really like this solution - it just works around the problem by
    > > 'speeding up' the system. If we have a _real_ slow system, there's no such
    > > way for us to speed it up.
    >
    >
    > It doesn' speed up the system actually. That's not the goal of this
    > patch.

    i meant that abstractly. The "solution" here is that your patch turns off
    the function-graph-tracer. The practical effect of that is: the system
    gets much faster at processing hrtimer IRQs and effectively "speeds up".

    ok?

    Ingo
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