Messages in this thread | | | Date | Sat, 3 Jun 2006 07:19:34 -0400 | From | Mathieu Desnoyers <> | Subject | Interrupts disabled for too long in printk |
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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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