[patch] mm, memcg: give exiting processes access to memory reserves

[patch] mm, memcg: give exiting processes access to memory reserves

[David Rientjes]

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@google.com>
---
 mm/memcontrol.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1686,11 +1686,11 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	struct task_struct *chosen = NULL;
 
 	/*
-	 * If current has a pending SIGKILL, then automatically select it.  The
-	 * goal is to allow it to allocate so that it may quickly exit and free
-	 * its memory.
+	 * If current has a pending SIGKILL or is exiting, then automatically
+	 * select it.  The goal is to allow it to allocate so that it may
+	 * quickly exit and free its memory.
 	 */
-	if (fatal_signal_pending(current)) {
+	if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
 		set_thread_flag(TIF_MEMDIE);
 		return;
 	}

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

[Michal Hocko]

On Wed 27-03-13 18:22:10, 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@google.com>

Acked-by: Michal Hocko <mhocko@suse.cz>

AFAIU this has been introduced by 9ff4868e (mm, oom: allow exiting
threads to have access to memory reserves) so maybe we want to mark it
for stable (3.8).

Thanks

> ---
>  mm/memcontrol.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1686,11 +1686,11 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	struct task_struct *chosen = NULL;
>  
>  	/*
> -	 * If current has a pending SIGKILL, then automatically select it.  The
> -	 * goal is to allow it to allocate so that it may quickly exit and free
> -	 * its memory.
> +	 * If current has a pending SIGKILL or is exiting, then automatically
> +	 * select it.  The goal is to allow it to allocate so that it may
> +	 * quickly exit and free its memory.
>  	 */
> -	if (fatal_signal_pending(current)) {
> +	if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
>  		set_thread_flag(TIF_MEMDIE);
>  		return;
>  	}

-- 
Michal Hocko
SUSE Labs

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

[Kamezawa Hiroyuki]

(2013/03/28 10:22), 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@google.com>


Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> ---
>   mm/memcontrol.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1686,11 +1686,11 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>   	struct task_struct *chosen = NULL;
>
>   	/*
> -	 * If current has a pending SIGKILL, then automatically select it.  The
> -	 * goal is to allow it to allocate so that it may quickly exit and free
> -	 * its memory.
> +	 * If current has a pending SIGKILL or is exiting, then automatically
> +	 * select it.  The goal is to allow it to allocate so that it may
> +	 * quickly exit and free its memory.
>   	 */
> -	if (fatal_signal_pending(current)) {
> +	if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
>   		set_thread_flag(TIF_MEMDIE);
>   		return;
>   	}
>


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

[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@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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