Re: [PATCH 1/2] mm,oom_reaper: Show trace of unable to reap victim thread.

[Michal Hocko <mhocko@suse.com>]

On Tue 20-03-18 22:50:21, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 20-03-18 22:30:16, Tetsuo Handa wrote:
> > > Michal Hocko wrote:
> > > > On Tue 20-03-18 21:52:33, Tetsuo Handa wrote:
> > > > > Michal Hocko wrote:
> > > > > > A single stack trace in the changelog would be sufficient IMHO.
> > > > > > Appart from that. What do you expect users will do about this trace?
> > > > > > Sure they will see a path which holds mmap_sem, we will see a bug report
> > > > > > but we can hardly do anything about that. We simply cannot drop the lock
> > > > > > from that path in 99% of situations. So _why_ do we want to add more
> > > > > > information to the log?
> > > > > 
> > > > > This case is blocked at i_mmap_lock_write().
> > > > 
> > > > But why does i_mmap_lock_write matter for oom_reaping. We are not
> > > > touching hugetlb mappings. dup_mmap holds mmap_sem for write which is
> > > > the most probable source of the backoff.
> > > 
> > > If i_mmap_lock_write can bail out upon SIGKILL, the OOM victim will be able to
> > > release mmap_sem held for write, which helps the OOM reaper not to back off.
> > 
> > There are so many other blocking calls (including allocations) in
> > dup_mmap 
> 
> Yes. But
> 
> >          that I do not really think i_mmap_lock_write is the biggest
> > problem. That will be likely the case for other mmap_sem write lockers.
> 
> i_mmap_lock_write() is one of the problems which we could afford fixing.
> 8 out of 11 "oom_reaper: unable to reap" messages are blocked at i_mmap_lock_write().
> 
[...]
> > Really I am not sure dumping more information is beneficial here.
> 
> Converting to use killable where we can afford is beneficial.

I am no questioning that. I am questioning the additional information
because we won't be able to do anything about mmap_sem holder most of
the time. Because they tend block on allocations...

-- 
Michal Hocko
SUSE Labs