Date:	Tue, 6 May 2014 16:00:32 +0200
From:	Jan Kara <jack@...e.cz>
To:	Will Deacon <will.deacon@....com>
Cc:	Jan Kara <jack@...e.cz>,
	"mm-commits@...r.kernel.org" <mm-commits@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"kay@...y.org" <kay@...y.org>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: +
 printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
 added to -mm tree

On Tue 06-05-14 14:12:34, Will Deacon wrote:
> On Tue, May 06, 2014 at 01:29:58PM +0100, Jan Kara wrote:
> > On Tue 06-05-14 13:06:48, Will Deacon wrote:
> > > On Fri, May 02, 2014 at 11:46:51PM +0100, Jan Kara wrote:
> > > >   I really dislike this patch. It goes completely against my efforts of
> > > > lowering irq latency caused by printing to console (which are the
> > > > problems I have observed ;).
> > > 
> > > Hmmm, what makes you think that? Interrupts only remain disabled whilst we
> > > process the backlog, which in the usual case should be pretty small. We also
> > > hold the logbuf_lock during this time, so we can't get stuck in an unbounded
> > > loop.
> > >
> > > Can you elaborate a bit more on the problems you've observed, please?
> >   Well, with serial console the backlog can get actually pretty big. During
> > boot on large machines I've seen CPUs stuck in that very loop in
> > console_unlock() for tens of seconds. Obviously that causes problems - e.g.
> > watchdog fires, RCU lockup detector fires, when interrupts are disabled,
> > some hardware gives up because its interrupts weren't served for too long.
> > All in all the machine just dies.
> 
> Right, so there's the usual compromise here between throughput and latency.
  I'd see that compromise if enabling & disabling interrupts would be
taking considerable amount of time. I don't think that was your concern,
was it? Maybe I just misunderstood you...

> I hoped that pegging the amount of irq-disabled work using console_end_seq
> would give us the best of both worlds, but you're saying that it can still
> lead to issues on large machines (in the worst case, the whole buffer fills
> up while we're printing the last lot, so the irq-disabled work amounts to
> draining logbuf, right?).
  Yup.

> That said, printing one message each time seems to go too far in the
> opposite direction for my liking, so the best bet is likely to limit the
> work to some fixed number of messages. Do you have any feeling for such a
> limit?
  If you really are concerned about enabling and disabling of interrupts
taking significant time (and it may be, I just don't know), then printing
couple of messages without enabling them makes sense. How many is a tricky
question since it depends on the console speed. I had a similar problem
when I was deciding in my patch when we should ask another CPU to take over
printing from the current CPU (to avoid the issues I've described in the
previous email). I was experimenting with various stuff but in the end I
restorted to a stupid "after X characters are printed".

> > And the backlog builds up because while one cpu is doing the printing in
> > console_unlock() all the other cpus are busily adding new messages to the
> > buffer faster than they can be printed...
> 
> Understood, but that's also the situation without this patch (and not one
> that I think you can fix without hurting latency).
  Sure. I have a patch which transitions printing to another CPU once in a
while so single CPU isn't hogged for too long and that solves the issues I
have observed. But Alan didn't like this solution so the issue is unfixed
for now.

> > > > My opinion is that when you are printing from each and every interrupt
> > > > which happens so often, then you have a problem and disabling IRQs in
> > > > printk so that your interrupt doesn't happen that often seems like a poor
> > > > solution to me. You could as well just ratelimit your debug messages,
> > > > couldn't you?
> > > 
> > > My use-case was basically using printk as a debug trace during early boot
> > > when bringing up Linux on a new CPU core. I don't think ratelimiting would
> > > be the right thing there, since I really did want as many messages to
> > > reach the console as possible (which is also why I wrote this patch, not
> > > just the other one in the series).
> >   OK, I understand. It just seems wrong to me to throttle all interrupts on
> > the cpu while doing printing just because someone might be doing debug
> > printing from the interrupt. Sure it's fine as a debug hack but on a
> > production machine that seems rather counterproductive.
> 
> Perhaps, but the one time I *really* want printk to be reliable is when I'm
> using it to debug a problem.
  Yes, I understand that. 

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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