Messages in this thread | | | Date | Sat, 6 Feb 2016 09:37:58 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH 5/5] mm, oom_reaper: implement OOM victims queuing |
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On Sat 06-02-16 14:54:24, Tetsuo Handa wrote: > Michal Hocko wrote: > > > But if we consider non system-wide OOM events, it is not very unlikely to hit > > > this race. This queue is useful for situations where memcg1 and memcg2 hit > > > memcg OOM at the same time and victim1 in memcg1 cannot terminate immediately. > > > > This can happen of course but the likelihood is _much_ smaller without > > the global OOM because the memcg OOM killer is invoked from a lockless > > context so the oom context cannot block the victim to proceed. > > Suppose mem_cgroup_out_of_memory() is called from a lockless context via > mem_cgroup_oom_synchronize() called from pagefault_out_of_memory(), that > "lockless" is talking about only current thread, doesn't it?
Yes and you need the OOM context to sit on the same lock as the victim to form a deadlock. So while the victim might be blocked somewhere it is much less likely it would be deadlocked.
> Since oom_kill_process() sets TIF_MEMDIE on first mm!=NULL thread of a > victim process, it is possible that non-first mm!=NULL thread triggers > pagefault_out_of_memory() and first mm!=NULL thread gets TIF_MEMDIE, > isn't it?
I got lost here completely. Maybe it is your usage of thread terminology again.
> Then, where is the guarantee that victim1 (first mm!=NULL thread in memcg1 > which got TIF_MEMDIE) is not waiting at down_read(&victim2->mm->mmap_sem) > when victim2 (first mm!=NULL thread in memcg2 which got TIF_MEMDIE) is > waiting at down_write(&victim2->mm->mmap_sem)
All threads/processes sharing the same mm are in fact in the same memory cgroup. That is the reason we have owner in the task_struct
> or both victim1 and victim2 > are waiting on a lock somewhere in memory reclaim path (e.g. > mutex_lock(&inode->i_mutex))?
Such waiting has to make a forward progress at some point in time because the lock itself cannot be deadlocked by the memcg OOM context.
-- Michal Hocko SUSE Labs
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