Re: [patch] mm, memcg: give exiting processes access to memoryreserves

[Johannes Weiner]

On Wed, Mar 27, 2013 at 06:22:10PM -0700, David Rientjes wrote:
> A memcg may livelock when oom if the process that grabs the hierarchy's
> oom lock is never the first process with PF_EXITING set in the memcg's
> task iteration.
> 
> The oom killer, both global and memcg, will defer if it finds an eligible
> process that is in the process of exiting and it is not being ptraced.
> The idea is to allow it to exit without using memory reserves before
> needlessly killing another process.
> 
> This normally works fine except in the memcg case with a large number of
> threads attached to the oom memcg.  In this case, the memcg oom killer
> only gets called for the process that grabs the hierarchy's oom lock; all
> others end up blocked on the memcg's oom waitqueue.  Thus, if the process
> that grabs the hierarchy's oom lock is never the first PF_EXITING process
> in the memcg's task iteration, the oom killer is constantly deferred
> without anything making progress.
> 
> The fix is to give PF_EXITING processes access to memory reserves so that
> we've marked them as oom killed without any iteration.  This allows
> __mem_cgroup_try_charge() to succeed so that the process may exit.  This
> makes the memcg oom killer exemption for TIF_MEMDIE tasks, now
> immediately granted for processes with pending SIGKILLs and those in the
> exit path, to be equivalent to what is done for the global oom killer.
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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