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@xxxxxxxxxxxxxxxxxxx> 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
