[PATCH v2 0/2] handle memoryless nodes more appropriately

[PATCH v2 0/2] handle memoryless nodes more appropriately

[Qi Zheng]

Hi all,

Currently, in the process of initialization or offline memory, memoryless
nodes will still be built into the fallback list of itself or other nodes.

This is not what we expected, so this patch series removes memoryless
nodes from the fallback list entirely.

This series is based on the next-20231018.

Comments and suggestions are welcome.

Thanks,
Qi

Changlog in v1 -> v2:
 - modify the commit message in [PATCH 1/2], mention that it can also fix the
   specific crash. (suggested by Ingo Molnar)

Qi Zheng (2):
  mm: page_alloc: skip memoryless nodes entirely
  mm: memory_hotplug: drop memoryless node from fallback lists

 mm/memory_hotplug.c | 2 +-
 mm/page_alloc.c     | 7 +++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

-- 
2.30.2

[PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

[Qi Zheng]

In find_next_best_node(), We skipped the memoryless nodes
when building the zonelists of other normal nodes (N_NORMAL),
but did not skip the memoryless node itself when building
the zonelist. This will cause it to be traversed at runtime.

For example, say we have node0 and node1, node0 is memoryless
node, then the fallback order of node0 and node1 as follows:

[    0.153005] Fallback order for Node 0: 0 1
[    0.153564] Fallback order for Node 1: 1

After this patch, we skip memoryless node0 entirely, then
the fallback order of node0 and node1 as follows:

[    0.155236] Fallback order for Node 0: 1
[    0.155806] Fallback order for Node 1: 1

So it becomes completely invisible, which will reduce runtime
overhead.

And in this way, we will not try to allocate pages from memoryless
node0, then the panic mentioned in [1] will also be fixed. Even though
this problem has been solved by dropping the NODE_MIN_SIZE constrain
in x86 [2], it would be better to fix it in core MM as well.

[1]. https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
[2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/page_alloc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ee392a324802..e978272699d3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5052,8 +5052,11 @@ int find_next_best_node(int node, nodemask_t *used_node_mask)
 	int min_val = INT_MAX;
 	int best_node = NUMA_NO_NODE;
 
-	/* Use the local node if we haven't already */
-	if (!node_isset(node, *used_node_mask)) {
+	/*
+	 * Use the local node if we haven't already. But for memoryless local
+	 * node, we should skip it and fallback to other nodes.
+	 */
+	if (!node_isset(node, *used_node_mask) && node_state(node, N_MEMORY)) {
 		node_set(node, *used_node_mask);
 		return node;
 	}
-- 
2.30.2

[PATCH v2 2/2] mm: memory_hotplug: drop memoryless node from fallback lists

[Qi Zheng]

In offline_pages(), if a node becomes memoryless, we
will clear its N_MEMORY state by calling node_states_clear_node().
But we do this after rebuilding the zonelists by calling
build_all_zonelists(), which will cause this memoryless node to
still be in the fallback list of other nodes. This will incur
some runtime overhead.

To drop memoryless node from fallback lists in this case, just
call node_states_clear_node() before calling build_all_zonelists().

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/memory_hotplug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index d4a364fdaf8f..18af399627f0 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -2036,12 +2036,12 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 	/* reinitialise watermarks and update pcp limits */
 	init_per_zone_wmark_min();
 
+	node_states_clear_node(node, &arg);
 	if (!populated_zone(zone)) {
 		zone_pcp_reset(zone);
 		build_all_zonelists(NULL);
 	}
 
-	node_states_clear_node(node, &arg);
 	if (arg.status_change_nid >= 0) {
 		kcompactd_stop(node);
 		kswapd_stop(node);
-- 
2.30.2

Re: [PATCH v2 0/2] handle memoryless nodes more appropriately

[David Hildenbrand]

On 19.10.23 09:36, Qi Zheng wrote:
> Hi all,
> 
> Currently, in the process of initialization or offline memory, memoryless
> nodes will still be built into the fallback list of itself or other nodes.
> 
> This is not what we expected, so this patch series removes memoryless
> nodes from the fallback list entirely.

What's the end result of this change -- IOW why do we care? Patch #1 
mentions "which will reduce runtime overhead." and patch #2 mentions 
"This will incur some runtime overhead.". IIUC the comment in patch #1 
correctly, these changes don't fix anything, correct?

Did you look into showing a performance gain?

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

[David Hildenbrand]

On 19.10.23 09:36, Qi Zheng wrote:
> In find_next_best_node(), We skipped the memoryless nodes
> when building the zonelists of other normal nodes (N_NORMAL),
> but did not skip the memoryless node itself when building
> the zonelist. This will cause it to be traversed at runtime.
> 
> For example, say we have node0 and node1, node0 is memoryless
> node, then the fallback order of node0 and node1 as follows:
> 
> [    0.153005] Fallback order for Node 0: 0 1
> [    0.153564] Fallback order for Node 1: 1
> 
> After this patch, we skip memoryless node0 entirely, then
> the fallback order of node0 and node1 as follows:
> 
> [    0.155236] Fallback order for Node 0: 1
> [    0.155806] Fallback order for Node 1: 1
> 
> So it becomes completely invisible, which will reduce runtime
> overhead.
> 
> And in this way, we will not try to allocate pages from memoryless
> node0, then the panic mentioned in [1] will also be fixed. Even though
> this problem has been solved by dropping the NODE_MIN_SIZE constrain
> in x86 [2], it would be better to fix it in core MM as well.
> 
> [1]. https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> [2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>   mm/page_alloc.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ee392a324802..e978272699d3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5052,8 +5052,11 @@ int find_next_best_node(int node, nodemask_t *used_node_mask)
>   	int min_val = INT_MAX;
>   	int best_node = NUMA_NO_NODE;
>   
> -	/* Use the local node if we haven't already */
> -	if (!node_isset(node, *used_node_mask)) {
> +	/*
> +	 * Use the local node if we haven't already. But for memoryless local
> +	 * node, we should skip it and fallback to other nodes.
> +	 */
> +	if (!node_isset(node, *used_node_mask) && node_state(node, N_MEMORY)) {
>   		node_set(node, *used_node_mask);
>   		return node;
>   	}

Makes sense to me; I suspect that online_pages() will just to the right 
thing and call build_all_zonelists() to fix it up.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 2/2] mm: memory_hotplug: drop memoryless node from fallback lists

[David Hildenbrand]

On 19.10.23 09:36, Qi Zheng wrote:
> In offline_pages(), if a node becomes memoryless, we
> will clear its N_MEMORY state by calling node_states_clear_node().
> But we do this after rebuilding the zonelists by calling
> build_all_zonelists(), which will cause this memoryless node to
> still be in the fallback list of other nodes. This will incur
> some runtime overhead.
> 
> To drop memoryless node from fallback lists in this case, just
> call node_states_clear_node() before calling build_all_zonelists().
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>   mm/memory_hotplug.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index d4a364fdaf8f..18af399627f0 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -2036,12 +2036,12 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>   	/* reinitialise watermarks and update pcp limits */
>   	init_per_zone_wmark_min();
>   
> +	node_states_clear_node(node, &arg);
>   	if (!populated_zone(zone)) {
>   		zone_pcp_reset(zone);
>   		build_all_zonelists(NULL);
>   	}
>   
> -	node_states_clear_node(node, &arg);
>   	if (arg.status_change_nid >= 0) {
>   		kcompactd_stop(node);
>   		kswapd_stop(node);

Probably worth a comment.

/*
  * Make sure to mark the node as memory-less before rebuilding the zone
  * list. Otherwise this node would still appear in the fallback lists.
  */

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 0/2] handle memoryless nodes more appropriately

[Qi Zheng]

Hi David,

On 2023/10/19 15:56, David Hildenbrand wrote:
> On 19.10.23 09:36, Qi Zheng wrote:
>> Hi all,
>>
>> Currently, in the process of initialization or offline memory, memoryless
>> nodes will still be built into the fallback list of itself or other 
>> nodes.
>>
>> This is not what we expected, so this patch series removes memoryless
>> nodes from the fallback list entirely.
> 
> What's the end result of this change -- IOW why do we care? Patch #1 
> mentions "which will reduce runtime overhead." and patch #2 mentions 
> "This will incur some runtime overhead.". IIUC the comment in patch #1 
> correctly, these changes don't fix anything, correct?

Yes, after dropping the NODE_MIN_SIZE constrain in x86, the panic
problem fixed by this patch no longer exists (Unless there are other
architectures that have this constrain).

The reason I am re-sending this patch is that I agree with Ingo's point
of view:

```
While I agree with dropping the limitation, and I agree that
9391a3f9c7f1 should have provided more of a justification, I believe a
core MM fix is in order as well, for it to not crash.
```

I also think that core MM should be safe (not crash) even in some weird
topology.

> 
> Did you look into showing a performance gain?
> 

No, and I think the performance gain should be small, after all it just
traverses one less node. ;)

Thanks,
Qi

Re: [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

[Qi Zheng]

Hi David,

On 2023/10/19 16:07, David Hildenbrand wrote:
> On 19.10.23 09:36, Qi Zheng wrote:
>> In find_next_best_node(), We skipped the memoryless nodes
>> when building the zonelists of other normal nodes (N_NORMAL),
>> but did not skip the memoryless node itself when building
>> the zonelist. This will cause it to be traversed at runtime.
>>
>> For example, say we have node0 and node1, node0 is memoryless
>> node, then the fallback order of node0 and node1 as follows:
>>
>> [    0.153005] Fallback order for Node 0: 0 1
>> [    0.153564] Fallback order for Node 1: 1
>>
>> After this patch, we skip memoryless node0 entirely, then
>> the fallback order of node0 and node1 as follows:
>>
>> [    0.155236] Fallback order for Node 0: 1
>> [    0.155806] Fallback order for Node 1: 1
>>
>> So it becomes completely invisible, which will reduce runtime
>> overhead.
>>
>> And in this way, we will not try to allocate pages from memoryless
>> node0, then the panic mentioned in [1] will also be fixed. Even though
>> this problem has been solved by dropping the NODE_MIN_SIZE constrain
>> in x86 [2], it would be better to fix it in core MM as well.
>>
>> [1]. 
>> https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
>> [2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   mm/page_alloc.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index ee392a324802..e978272699d3 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5052,8 +5052,11 @@ int find_next_best_node(int node, nodemask_t 
>> *used_node_mask)
>>       int min_val = INT_MAX;
>>       int best_node = NUMA_NO_NODE;
>> -    /* Use the local node if we haven't already */
>> -    if (!node_isset(node, *used_node_mask)) {
>> +    /*
>> +     * Use the local node if we haven't already. But for memoryless 
>> local
>> +     * node, we should skip it and fallback to other nodes.
>> +     */
>> +    if (!node_isset(node, *used_node_mask) && node_state(node, 
>> N_MEMORY)) {
>>           node_set(node, *used_node_mask);
>>           return node;
>>       }
> 
> Makes sense to me; I suspect that online_pages() will just to the right 
> thing and call build_all_zonelists() to fix it up.

Yes, the find_next_best_node() will be called by build_all_zonelists().

> 
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks.

> 

Re: [PATCH v2 2/2] mm: memory_hotplug: drop memoryless node from fallback lists

[Qi Zheng]

Hi David,

On 2023/10/19 16:11, David Hildenbrand wrote:
> On 19.10.23 09:36, Qi Zheng wrote:
>> In offline_pages(), if a node becomes memoryless, we
>> will clear its N_MEMORY state by calling node_states_clear_node().
>> But we do this after rebuilding the zonelists by calling
>> build_all_zonelists(), which will cause this memoryless node to
>> still be in the fallback list of other nodes. This will incur
>> some runtime overhead.
>>
>> To drop memoryless node from fallback lists in this case, just
>> call node_states_clear_node() before calling build_all_zonelists().
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   mm/memory_hotplug.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index d4a364fdaf8f..18af399627f0 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -2036,12 +2036,12 @@ int __ref offline_pages(unsigned long 
>> start_pfn, unsigned long nr_pages,
>>       /* reinitialise watermarks and update pcp limits */
>>       init_per_zone_wmark_min();
>> +    node_states_clear_node(node, &arg);
>>       if (!populated_zone(zone)) {
>>           zone_pcp_reset(zone);
>>           build_all_zonelists(NULL);
>>       }
>> -    node_states_clear_node(node, &arg);
>>       if (arg.status_change_nid >= 0) {
>>           kcompactd_stop(node);
>>           kswapd_stop(node);
> 
> Probably worth a comment.
> 
> /*
>   * Make sure to mark the node as memory-less before rebuilding the zone
>   * list. Otherwise this node would still appear in the fallback lists.
>   */

OK, will add it in the v3.

> 
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks.

>