Messages in this thread | | | Date | Wed, 12 Mar 2014 16:06:17 +0100 | From | Peter Zijlstra <> | Subject | Re: is printk() safe within a timekeeper_seq write section? |
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On Wed, Mar 12, 2014 at 03:34:56PM +0100, Jan Kara wrote: > On Wed 12-03-14 07:46:45, Peter Zijlstra wrote: > > On Tue, Mar 11, 2014 at 10:32:26PM +0100, Thomas Gleixner wrote: > > > > Peter/Thomas: Any thoughts on the deferred printk buffer? Does printk > > > > already have something like this? Any other ideas here? > > > > > > I was thinking about something like that for RT as on RT printk is a > > > complete nightmare. It's simple to implement that, but as we know from > > > the RT experience it can lead to painful loss of debug output. > > > > > > Assume you printk inside such a region, which just fills the dmesg > > > buffer and schedules the delayed output. Now in that same region you > > > run into a deadlock which causes the whole machine to freeze. Then you > > > won't see the debug output, which might actually give you the hint why > > > the system deadlocked .... > > > > Ok so I started writing a rant that I don't give a crap about klogd and > > that deferring that wakeup would be perfectly fine; then I looked at the > > code and found that we in fact do this already. > > > > wake_up_klogd() schedules a lazy irqwork to go wake up, so that's out. > > > > That leaves the console sem wakeup; but I suppose we could redo this > > patch: > > > > lkml.kernel.org/r/20110621153806.286257129@chello.nl > > > > to get rid of that one. > I don't know if you've noticed but there's also the following patch: > https://lkml.org/lkml/2013/12/23/310 > which would make it pretty easy to just add messages to printk buffer in > timer code and schedule printing later using irq work.
Yeah; I suppose that one is prettier.
> Regarding your referenced patch - the way it is written, it would make > all printk users spin on console_sem->lock all the time while we are > flushing buffer to console. I don't think we want that - we trylock the > console_sem exactly so that other printk users can proceed while one poor > guy is pushing stuff to console.
That should be fixable though; just keep enough state for the other printk()s to see they don't need to also flush.
But the idea is to not do the sleep+wakeup dance.
But as stated; that's not going to actually matter much, since the popular console drivers are crap and do wakeups too.
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