Re: [PATCH] mm, oom: avoid printk() iteration under RCU

[Michal Hocko <mhocko@suse.com>]

On Sun 22-09-19 08:47:31, Tetsuo Handa wrote:
> On 2019/09/22 5:30, Michal Hocko wrote:
> > On Fri 20-09-19 17:10:42, Andrew Morton wrote:
> >> On Sat, 20 Jul 2019 20:29:23 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:
> >>
> >>>>
> >>>>> ) under RCU and this patch is one of them (except that we can't remove
> >>>>> printk() for dump_tasks() case).
> >>>>
> >>>> No, this one adds a complexity for something that is not clearly a huge
> >>>> win or the win is not explained properly.
> >>>>
> >>>
> >>> The win is already explained properly by the past commits. Avoiding RCU stalls
> >>> (even without slow consoles) is a clear win. The duration of RCU stall avoided
> >>> by this patch is roughly the same with commit b2b469939e934587.
> >>>
> >>> We haven't succeeded making printk() asynchronous (and potentially we won't
> >>> succeed making printk() asynchronous because we need synchronous printk()
> >>> when something critical is undergoing outside of out_of_memory()). Thus,
> >>> bringing printk() to outside of RCU section is a clear win we can make for now.
> >>
> >> It's actually not a complex patch and moving all that printing outside
> >> the rcu section makes sense.  So I'll sit on the patch for a few more
> >> days but am inclined to send it upstream.
> > 
> > Look, I am quite tired of arguing about this and other changes following
> > the similar pattern. In short a problematic code is shuffled around and
> > pretend to solve some problem. In this particular case it is a RCU stall
> > which in itself is not a fatal condition. Sure it sucks and the primary
> > reason is that printk can take way too long. This is something that is
> > currently a WIP to be address. What is more important though there is no
> > sign of any _real world_ workload that would require a quick workaround
> > to justify a hacky stop gap solution.
> > 
> > So again, why do we want to add more code for something which is not
> > clear to be a real life problem and that will add a maintenance burden
> > for future?
> > 
> 
> Enqueueing zillion printk() lines from dump_tasks() will overflow printk
> buffer (i.e. leads to lost messages) if OOM killer messages were printed
> asynchronously. I don't think that making printk() asynchronous will solve
> this problem. I repeat again; there is no better solution than "printk()
> users are careful not to exhaust the printk buffer". This patch is the
> first step towards avoiding thoughtless printk().

Irrelevant because this patch doesn't reduce the amount of output.

> Delay from dump_tasks() not only affects a thread holding oom_lock but also
> other threads which are directly doing concurrent allocation requests or
> indirectly waiting for the thread holding oom_lock. Your "it is a RCU stall
> which in itself is not a fatal condition" is underestimating the _real world_
> problems (e.g. "delay can trigger watchdog timeout and cause the system to
> reboot even if the administrator does not want the system to reboot").

Please back your claims by real world examples.
-- 
Michal Hocko
SUSE Labs