Re: [PATCH] net/skbuff: silence warnings under memory pressure

On Wed, 2019-09-04 at 10:25 +0200, Michal Hocko wrote:

On Wed 04-09-19 16:00:42, Sergey Senozhatsky wrote:

quoted

On (09/04/19 15:41), Sergey Senozhatsky wrote:

quoted

But the thing is different in case of dump_stack() + show_mem() +
some other output. Because now we ratelimit not a single printk() line,
but hundreds of them. The ratelimit becomes - 10 * $$$ lines in 5 seconds
(IOW, now we talk about thousands of lines).

And on devices with slow serial consoles this can be somewhat close to
"no ratelimit". *Suppose* that warn_alloc() adds 700 lines each time.
Within 5 seconds we can call warn_alloc() 10 times, which will add 7000
lines to the logbuf. If printk() can evict only 6000 lines in 5 seconds
then we have a growing number of pending logbuf messages.

Yes, ratelimit is problematic when the ratelimited operation is slow. I
guess that is a well known problem and we would need to rework both the
api and the implementation to make it work in those cases as well.
Essentially we need to make the ratelimit act as a gatekeeper to an
operation section - something like a critical section except you can
tolerate more code executions but not too many. So effectively

	start_throttle(rate, number);
	/* here goes your operation */
	end_throttle();

one operation is not considered done until the whole section ends.
Or something along those lines.

In this particular case we can increase the rate limit parameters of
course but I think that longterm we need a better api.

The problem is when a system is under heavy memory pressure, everything is
becoming slower, so I don't know how to come up with a sane default for rate
limit parameters as a generic solution that would work for every machine out
there. Sure, it is possible to set a limit as low as possible that would work
for the majority of systems apart from people may complain that they are now
missing important warnings, but using __GFP_NOWARN in this code would work for
all systems. You could even argument there is even a separate benefit that it
could reduce the noise-level overall from those build_skb() allocation failures
as it has a fall-back mechanism anyway.