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    SubjectInterrupts disabled for too long in printk
    Hi,

    I ran some experiments with my kernel tracer (LTTng : http://ltt.polymtl.ca)
    that showed missing interrupts. I wrote a small paper to show how to use my
    tracer to solve this kind of problem which I presented at the CE Linux Form
    last April.

    http://tree.celinuxforum.org/CelfPubWiki/ELC2006Presentations?action=AttachFile&do=get&target=celf2006-desnoyers.pdf

    It shows that, when the serial console is activated, the following code disables
    interrupts for up to 15ms. On a system configured with a 250HZ timer (each 4ms),
    it means that 3 scheduler ticks are lost.

    In the current git :

    kernel/printk.c: release_console_sem()

    for ( ; ; ) {
    -----> spin_lock_irqsave(&logbuf_lock, flags);
    wake_klogd |= log_start - log_end;
    if (con_start == log_end)
    break; /* Nothing to print */
    _con_start = con_start;
    _log_end = log_end;
    con_start = log_end; /* Flush */
    spin_unlock(&logbuf_lock);
    call_console_drivers(_con_start, _log_end);
    -----> local_irq_restore(flags);
    }

    I guess interrupts are disabled for a good reason (to protect this spinlock for
    being taken by a nested interrupt handler. One way I am thinking to fix this
    problem would be to do a spin try lock and fail if it is already taken.

    Mathieu


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