Re: [PATCH] printk: Ratelimit messages printed by console drivers

From: Petr Mladek
Date: Wed May 09 2018 - 08:01:00 EST

On Fri 2018-04-27 19:22:45, Sergey Senozhatsky wrote:
> On (04/26/18 11:42), Petr Mladek wrote:
> [..]
> > Honestly, I do not believe that console drivers are like Scheherazade.
> > They are not able to make up long interesting stories. Let's say that
> > lockdep splat has more than 100 lines but it can happen only once.
> > Let's say that WARNs have about 40 lines. I somehow doubt that we
> > could ever see 10 different WARN calls from one con->write() call.
> The problem here is that it takes a human being with IQ to tell what's
> repetitive, what's useless and what's not.
> vprintk(...)
> {
> if (!__ratelimit())
> return;
> }
> has zero IQ to make such decisions.

You make it too complicated. Also it seems that you repeatedly
hide the fact that con->write() context is recursive. Just try to add
printk() into call_console_drivers() and see what happens.

IMHO, if con->write() wants to add more than 1000 (or 100 or whatever
sane limit) new lines then something is really wrong and we should
stop it. It is that simple.

> > > But we first need a real reason. Right now it looks to me like
> > > we have "a solution" to a problem which we have never witnessed.
> >
> > I am trying to find a "simple" and generic solution for the problem
> > reported by Tejun:
> [..]
> > 1. Console is IPMI emulated serial console. Super slow. Also
> > netconsole is in use.
> > 2. System runs out of memory, OOM triggers.
> > 3. OOM handler is printing out OOM debug info.
> > 4. While trying to emit the messages for netconsole, the network stack
> > / driver tries to allocate memory and then fail, which in turn
> > triggers allocation failure or other warning messages. printk was
> > already flushing, so the messages are queued on the ring.
> > 5. OOM handler keeps flushing but 4 repeats and the queue is never
> > shrinking. Because OOM handler is trapped in printk flushing, it
> > never manages to free memory and no one else can enter OOM path
> > either, so the system is trapped in this state.
> > </paste>

IMHO, we do not need to chase down this particular problem. It was
already "solved" by the commit 400e22499dd92613821 ("mm: don't warn
about allocations which stall for too long").

It was just an example. I wanted to make con->write() generally safe.
I thought that the problem (recursion) was clear enough.

> Yes, and that's why I want to take a look at the logs/backtraces.

If you want more cases to analyze, fair enough. I do not have any
at hands. It is not an urgent issue for me and I am not going to
spend more time on this.

Best Regards,