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    SubjectRe: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace

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

    > > [<ffffffff8020c79d>] return_to_handler+0x0/0x73
    > > [<ffffffff8029ea13>] rcu_pending+0x2c/0x5e
    > > [<ffffffff8020c79d>] return_to_handler+0x0/0x73
    > > [<ffffffff8026abef>] update_process_times+0x3c/0x77
    > > [<ffffffff8020c79d>] return_to_handler+0x0/0x73
    > > [<ffffffff802875dd>] tick_periodic+0x6e/0x70
    >
    >
    > Still hanging in the timer interrupt.
    > I guess it makes the timer interrupt servicing too slow and then
    > once it is serviced, another one is raised.
    >
    > But the cause is perhaps more complex
    >
    > I think you have had too much hanging of this type. I'm preparing
    > a fix that checks periodically if the function graph tracer is
    > spending too much time in an interrupt.
    >
    > I guess I could count the number of function executed between the
    > irq entry and its exit.
    >
    > That's the best: if we are hanging in an interrupt, it could be
    > whatever interrupt and the jiffies could not be progressing so I
    > can't rely on time but only on number of functions executed.
    >
    > May be 10000 calls is a good threshold before killing the function
    > graph inside an interrupt?

    i think the problem isnt even the IRQ handler - but the fact that
    the (timer) irq handler gets re-triggered - so all we do is
    processing timer IRQs.

    Your patch would detect a timer IRQ hanging - but it would not
    detect the 'system makes no progress because there's always anoter
    pending timer IRQ to execute' situation.

    So i think we need a "function trace watchdog" - which kills the
    tracer if we do more than 100,000,000 entries since we started the
    self-test, or so.

    Ingo


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