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    SubjectRe: CONFIG_PREEMPT and server workloads
    At Thu, 18 Mar 2004 11:01:59 -0800,
    Andrew Morton wrote:
    > btw, several months ago we discussed the idea of adding a sysctl to the
    > ALSA drivers which would cause a dump_stack() to be triggered if the audio
    > ISR detected a sound underrun.
    > This would be a very useful feature, because it increases the number of
    > low-latency developers from O(2) to O(lots). If some user is complaining
    > of underruns we can just ask them to turn on the sysctl and we get a trace
    > pointing at the culprit code.
    > And believe me, we need the coverage. There are all sorts of weird code
    > paths which were found during the development of the 2.4 low-latency patch.
    > i2c drivers, fbdev drivers, all sorts of things which you and I don't
    > test.
    > I know it's a matter of
    > if (sysctl_is_set)
    > dump_stack();
    > in snd_pcm_update_hw_ptr_post() somewhere, but my brain burst when working
    > out the ALSA sysctl architecture.
    > Is this something you could add please?

    oh, sorry, maybe i forgot to tell you that it has been already there

    # echo 1 > /proc/asound/card0/pcm0p/xrun_debug

    this will show the stacktrace when a buffer overrun/underrun is
    detected in the irq handler. it's not perfect, though.

    we can add stacktracing in other nasty places, e.g. when the
    unexpected h/w pointer is returned (this is usually because of sloppy
    irq handling).

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