[PATCH v2 1/2] memcg, oom: remove unnecessary check in mem_cgroup_oom_synchronize()

[PATCH v2 1/2] memcg, oom: remove unnecessary check in mem_cgroup_oom_synchronize()

[Haifeng Xu]

mem_cgroup_oom_synchronize() is only used when the memcg oom handling is
handed over to the edge of the #PF path. Since commit 29ef680ae7c2 ("memcg,
oom: move out_of_memory back to the charge path") this is the case only
when the kernel memcg oom killer is disabled (current->memcg_in_oom is
only set if memcg->oom_kill_disable). Therefore a check for
oom_kill_disable in mem_cgroup_oom_synchronize() is not required.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
---
v2: split original into two and improve patch description
---
 mm/memcontrol.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5abffe6f8389..fbf4d2bb1003 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1999,16 +1999,9 @@ bool mem_cgroup_oom_synchronize(bool handle)
 	if (locked)
 		mem_cgroup_oom_notify(memcg);
 
-	if (locked && !memcg->oom_kill_disable) {
-		mem_cgroup_unmark_under_oom(memcg);
-		finish_wait(&memcg_oom_waitq, &owait.wait);
-		mem_cgroup_out_of_memory(memcg, current->memcg_oom_gfp_mask,
-					 current->memcg_oom_order);
-	} else {
-		schedule();
-		mem_cgroup_unmark_under_oom(memcg);
-		finish_wait(&memcg_oom_waitq, &owait.wait);
-	}
+	schedule();
+	mem_cgroup_unmark_under_oom(memcg);
+	finish_wait(&memcg_oom_waitq, &owait.wait);
 
 	if (locked) {
 		mem_cgroup_oom_unlock(memcg);
-- 
2.25.1

Re: [PATCH v2 1/2] memcg, oom: remove unnecessary check in mem_cgroup_oom_synchronize()

[Michal Hocko]

On Wed 19-04-23 03:07:38, Haifeng Xu wrote:
> mem_cgroup_oom_synchronize() is only used when the memcg oom handling is
> handed over to the edge of the #PF path. Since commit 29ef680ae7c2 ("memcg,
> oom: move out_of_memory back to the charge path") this is the case only
> when the kernel memcg oom killer is disabled (current->memcg_in_oom is
> only set if memcg->oom_kill_disable). Therefore a check for
> oom_kill_disable in mem_cgroup_oom_synchronize() is not required.
> 
> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>

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

> ---
> v2: split original into two and improve patch description
> ---
>  mm/memcontrol.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..fbf4d2bb1003 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1999,16 +1999,9 @@ bool mem_cgroup_oom_synchronize(bool handle)
>  	if (locked)
>  		mem_cgroup_oom_notify(memcg);
>  
> -	if (locked && !memcg->oom_kill_disable) {
> -		mem_cgroup_unmark_under_oom(memcg);
> -		finish_wait(&memcg_oom_waitq, &owait.wait);
> -		mem_cgroup_out_of_memory(memcg, current->memcg_oom_gfp_mask,
> -					 current->memcg_oom_order);
> -	} else {
> -		schedule();
> -		mem_cgroup_unmark_under_oom(memcg);
> -		finish_wait(&memcg_oom_waitq, &owait.wait);
> -	}
> +	schedule();
> +	mem_cgroup_unmark_under_oom(memcg);
> +	finish_wait(&memcg_oom_waitq, &owait.wait);
>  
>  	if (locked) {
>  		mem_cgroup_oom_unlock(memcg);
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs

[PATCH v2 2/2] memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()

[Haifeng Xu]

Before commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to
the charge path"), all memcg oom killers were delayed to page fault
path. And the explicit wakeup is used in this case:

thread A:
        ...
        if (locked) {           // complete oom-kill, hold the lock
                mem_cgroup_oom_unlock(memcg);
                ...
        }
        ...

thread B:
        ...

        if (locked && !memcg->oom_kill_disable) {
                ...
        } else {
                schedule();     // can't acquire the lock
                ...
        }
        ...

The reason is that thread A kicks off the OOM-killer, which leads to
wakeups from the uncharges of the exiting task. But thread B is not
guaranteed to see them if it enters the OOM path after the OOM kills
but before thread A releases the lock.

Now only oom_kill_disable case is handled from the #PF path. In that
case it is userspace to trigger the wake up not the #PF path itself.
All potential paths to free some charges are responsible to call
memcg_oom_recover() , so the explicit wakeup is not needed in the
mem_cgroup_oom_synchronize() path which doesn't release any memory
itself.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
---
v2: split original into two and improve patch description
---
 mm/memcontrol.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fbf4d2bb1003..710ce3e7824f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2003,15 +2003,8 @@ bool mem_cgroup_oom_synchronize(bool handle)
 	mem_cgroup_unmark_under_oom(memcg);
 	finish_wait(&memcg_oom_waitq, &owait.wait);
 
-	if (locked) {
+	if (locked)
 		mem_cgroup_oom_unlock(memcg);
-		/*
-		 * There is no guarantee that an OOM-lock contender
-		 * sees the wakeups triggered by the OOM kill
-		 * uncharges.  Wake any sleepers explicitly.
-		 */
-		memcg_oom_recover(memcg);
-	}
 cleanup:
 	current->memcg_in_oom = NULL;
 	css_put(&memcg->css);
-- 
2.25.1

Re: [PATCH v2 2/2] memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()

[Michal Hocko]

On Wed 19-04-23 03:07:39, Haifeng Xu wrote:
> Before commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to
> the charge path"), all memcg oom killers were delayed to page fault
> path. And the explicit wakeup is used in this case:
> 
> thread A:
>         ...
>         if (locked) {           // complete oom-kill, hold the lock
>                 mem_cgroup_oom_unlock(memcg);
>                 ...
>         }
>         ...
> 
> thread B:
>         ...
> 
>         if (locked && !memcg->oom_kill_disable) {
>                 ...
>         } else {
>                 schedule();     // can't acquire the lock
>                 ...
>         }
>         ...
> 
> The reason is that thread A kicks off the OOM-killer, which leads to
> wakeups from the uncharges of the exiting task. But thread B is not
> guaranteed to see them if it enters the OOM path after the OOM kills
> but before thread A releases the lock.
> 
> Now only oom_kill_disable case is handled from the #PF path. In that
> case it is userspace to trigger the wake up not the #PF path itself.
> All potential paths to free some charges are responsible to call
> memcg_oom_recover() , so the explicit wakeup is not needed in the
> mem_cgroup_oom_synchronize() path which doesn't release any memory
> itself.
> 
> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>

I hope I haven't missed anything but this looks good to me. One way to
test this would be a parallel OOM triggering workload which uses page
faults and an automatic user space driven oom killer (detect under_oom
and send SIGKILL to a random task after a random timeout). Objective is
that no task gets stuck in mem_cgroup_oom_synchronize. I am pretty sure
this could be easily turned into a selftest.

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

> ---
> v2: split original into two and improve patch description
> ---
>  mm/memcontrol.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fbf4d2bb1003..710ce3e7824f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2003,15 +2003,8 @@ bool mem_cgroup_oom_synchronize(bool handle)
>  	mem_cgroup_unmark_under_oom(memcg);
>  	finish_wait(&memcg_oom_waitq, &owait.wait);
>  
> -	if (locked) {
> +	if (locked)
>  		mem_cgroup_oom_unlock(memcg);
> -		/*
> -		 * There is no guarantee that an OOM-lock contender
> -		 * sees the wakeups triggered by the OOM kill
> -		 * uncharges.  Wake any sleepers explicitly.
> -		 */
> -		memcg_oom_recover(memcg);
> -	}
>  cleanup:
>  	current->memcg_in_oom = NULL;
>  	css_put(&memcg->css);
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs