Re: IRQ off latency of printk is very high

Mathieu Desnoyers
2007-11-01T22:19:20Z

* Pavel Machek (pavel@ucw.cz) wrote:
> Hi!
>
> > > > It seems draconian to drain the entire buffer with ints disabled.
> > > > Is it possible to break this up and send out smaller chunks
> > > > at a time? Maybe by putting a chunk loop in release_console_sem()?
> > >
> > > Well, I believe someone got
> > >
> > > DDetetccctted ed 113223 HHzz CPUCPU
> > >
> > > in his dmesg, and now we have this 'draconian' locking. How can we
> > > prevent mangled messages without it?
> >
> > The main interest seems to be to protect from mixed printk output
> > between different CPUs in process context. I don't think it would be
> > that bad if interrupts come and output error messages in the middle of a
> > printk, isn't it ?
> >
> > therefore, could we do something like :
> >
> >
> > if (!in_irq())
> > spin_lock(&logbuf_lock);
> > ...
> > if (!in_irq())
> > spin_unlock(&logbuf_lock);
> >
> > ? (yes, this is a crazy idea)
>
> Two messages in atomic sections on different cpus could still be mixed
> :-). But yes, something like this may be the way to go.

Not in "preempt disable" sections though. Only in interrupt handlers.
But yes, I assume here that messages coming from interrupt handlers can
afford being interleaved.

Mathieu

> Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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