mm: don't raise MEMCG_OOM event due to failed high-order allocation

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

In this case the idea of raising the MEMCG_OOM event looks dubious.

Fix this by moving MEMCG_OOM raising to mem_cgroup_oom() after allocation order check, so that the event won't be raised for high order allocations.

Signed-off-by Roman Gushchin <[email protected]>

Cc Johannes Weiner <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Vladimir Davydov <[email protected]>

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

In this case the idea of raising the MEMCG_OOM event looks dubious.

Fix this by moving MEMCG_OOM raising to mem_cgroup_oom() after allocation order check, so that the event won't be raised for high order allocations.

Signed-off-by Roman Gushchin <[email protected]>

Cc Johannes Weiner <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Vladimir Davydov <[email protected]>

---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

mm/memcontrol.c +2 -2

index fcec9b39e2a3..103ca3c31c04 100644
@@ -1669,6 +1669,8 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
 	if (order > PAGE_ALLOC_COSTLY_ORDER)
 		return OOM_SKIPPED;
 
+	memcg_memory_event(memcg, MEMCG_OOM);
+
 	/*
 	 * We are in the middle of the charge context here, so we
 	 * don't want to block when potentially sitting on a callstack

@@ -2250,8 +2252,6 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	if (fatal_signal_pending(current))
 		goto force;
 
-	memcg_memory_event(mem_over_limit, MEMCG_OOM);
-
 	/*
 	 * keep retrying as long as the memcg oom killer is able to make
 	 * a forward progress or bypass the charge if the oom killer

Signature · 1 line

2.17.1

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Quoted 11 lines · Michal Hocko

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

Quoted 5 lines

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Why not introduce a MEMCG_ALLOC_FAIL in to memcg_memory_event?

Quoted 11 lines · Michal Hocko

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

Quoted 5 lines

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Why not introduce a MEMCG_ALLOC_FAIL in to memcg_memory_event?

Quoted 14 lines · Roman Gushchin

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

Quoted 8 lines

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

In this case the idea of raising the MEMCG_OOM event looks dubious.

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Quoted 32 lines · Michal Hocko

Quoted 14 lines · Roman Gushchin

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

Quoted 8 lines

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

In this case the idea of raising the MEMCG_OOM event looks dubious.

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Quoted 7 lines

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Right, the current behavior matches the documentation, because the description of the event is broad enough. My point is that the current behavior is not useful in my corner case.

Let me explain my case in details: I've got a report about sporadic memcg oom kills on some hosts with plenty of pagecache and low memory pressure. You'll probably agree, that raising OOM signal in this case looks strange.

Quoted 45 lines · Roman Gushchin

Quoted 32 lines · Michal Hocko

Quoted 14 lines · Roman Gushchin

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

Quoted 8 lines

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

In this case the idea of raising the MEMCG_OOM event looks dubious.

Why is this a problem though? IIRC this event was deliberately placed outside of the oom path because we wanted to count allocation failures and this is also documented that way

oom
The number of time the cgroup's memory usage was reached the limit and allocation was about to fail.

Quoted 7 lines

Depending on context result could be invocation of OOM killer and retrying allocation or failing a

One could argue that we do not apply the same logic to GFP_NOWAIT requests but in general I would like to see a good reason to change the behavior and if it is really the right thing to do then we need to update the documentation as well.

Right, the current behavior matches the documentation, because the description of the event is broad enough. My point is that the current behavior is not useful in my corner case.

Quoted 3 lines

Let me explain my case in details: I've got a report about sporadic memcg oom kills on some hosts with plenty of pagecache and low memory pressure. You'll probably agree, that raising OOM signal in this case looks strange.

I am not sure I follow. So you see both OOM_KILL and OOM events and the user misinterprets OOM ones?

My understanding was that OOM event should tell admin that the limit should be increased in order to allow more charges. Without OOM_KILL events it means that those failed charges have some sort of fallback so it is not critical condition for the workload yet. Something to watch for though in case of perf. degradation or potential misbehavior.

Whether this is how the event is used, I dunno. Anyway, if you want to just move the event and make it closer to OOM_KILL then I strongly suspect the event is losing its relevance.

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

Quoted 2 lines · Roman Gushchin

The memcg OOM killer is never invoked due to a failed high-order allocation, however the MEMCG_OOM event can be easily raised.

Wasn't the same also true for kernel allocations until recently? We'd signal MEMCG_OOM and then return -ENOMEM.

Quoted 12 lines

Under some memory pressure it can happen easily because of a concurrent allocation. Let's look at try_charge(). Even if we were able to reclaim enough memory, this check can fail due to a race with another allocation:

if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
goto retry;

For regular pages the following condition will save us from triggering the OOM:

Quoted 6 lines

   if (nr_reclaimed && nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER))
       goto retry;

But for high-order allocation this condition will intentionally fail. The reason behind is that we'll likely fall to regular pages anyway, so it's ok and even preferred to return ENOMEM.

These seem to be more implementation details than anything else.

Personally, I'm confused by the difference between the "oom" and "oom_kill" events, and I don't understand when you would be interested in one and when in the other. The difference again seems to be mostly implementation details.

But the definition of "oom"/MEMCG_OOM in cgroup-v2.rst applies to the situation of failing higher-order allocations. I'm not per-se against changing the semantics here, as I don't think they are great. But can you please start out with rewriting the definition in a way that shows the practical difference for users?

The original idea behind MEMCG_OOM was to signal when reclaim had failed and we defer to the oom killer. The oom killer may or may not kill anything, which is the case for higher order allocations, but that doesn't change the out-of-memory situation that has occurred.

Konstantin added the OOM_KILL events to count actual kills. It seems to me that this has much more practical applications than the more theoretical OOM, since users care more about kills and not necessarily about "reclaim failed (but i might have been able to handle it with retries and fallback allocations, and so there isn't an actual issue".

Is there a good reason for keeping OOM now that we have OOM_KILL?