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

[Michal Hocko <mhocko@suse.com>]

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?
-- 
Michal Hocko
SUSE Labs