Re: + mm-page_alloc-fix-numa-stats-update-for-cpu-less-nodes.patch added to mm-hotfixes-unstable branch

[Michal Hocko <mhocko@suse.com>]

I believe this patch should be dropped. Not only it doesn't really
describes an actual problem I believe it is indeed incorrect as
explained https://lore.kernel.org/all/ZxoEWBakAv64wfhD@tiehlicka/T/#u

On Wed 23-10-24 13:41:25, Andrew Morton wrote:
> From: Dongjoo Seo <dongjoo.linux.dev@gmail.com>
> Subject: mm/page_alloc: fix NUMA stats update for cpu-less nodes
> Date: Wed, 23 Oct 2024 10:50:37 -0700
> 
> In the case of memoryless node, when a process prefers a node with no
> memory(e.g., because it is running on a CPU local to that node), the
> kernel treats a nearby node with memory as the preferred node.  As a
> result, such allocations do not increment the numa_foreign counter on the
> memoryless node, leading to skewed NUMA_HIT, NUMA_MISS, and NUMA_FOREIGN
> stats for the nearest node.
> 
> This patch corrects this issue by:
> 1. Checking if the zone or preferred zone is CPU-less before updating
>    the NUMA stats.
> 2. Ensuring NUMA_HIT is only updated if the zone is not CPU-less.
> 3. Ensuring NUMA_FOREIGN is only updated if the preferred zone is not
>    CPU-less.
> 
> Example Before and After Patch:
> - Before Patch:
>  node0                   node1           node2
>  numa_hit                86333181       114338269            5108
>  numa_miss                5199455               0        56844591
>  numa_foreign            32281033        29763013               0
>  interleave_hit                91              91               0
>  local_node              86326417       114288458               0
>  other_node               5206219           49768        56849702
> 
> - After Patch:
>                             node0           node1           node2
>  numa_hit                 2523058         9225528               0
>  numa_miss                 150213           10226        21495942
>  numa_foreign            17144215         4501270               0
>  interleave_hit                91              94               0
>  local_node               2493918         9208226               0
>  other_node                179351           27528        21495942
> 
> Similarly, in the context of cpuless nodes, this patch ensures that NUMA
> statistics are accurately updated by adding checks to prevent the
> miscounting of memory allocations when the involved nodes have no CPUs. 
> This ensures more precise tracking of memory access patterns accross all
> nodes, regardless of whether they have CPUs or not, improving the overall
> reliability of NUMA stat.  The reason is that page allocation from
> dev_dax, cpuset, memcg ..  comes with preferred allocating zone in cpuless
> node and its hard to track the zone info for miss information.
> 
> Link: https://lkml.kernel.org/r/20241023175037.9125-1-dongjoo.linux.dev@gmail.com
> Signed-off-by: Dongjoo Seo <dongjoo.linux.dev@gmail.com>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: Fan Ni <nifan@outlook.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Adam Manzanares <a.manzanares@samsung.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/page_alloc.c |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> --- a/mm/page_alloc.c~mm-page_alloc-fix-numa-stats-update-for-cpu-less-nodes
> +++ a/mm/page_alloc.c
> @@ -2858,19 +2858,21 @@ static inline void zone_statistics(struc
>  {
>  #ifdef CONFIG_NUMA
>  	enum numa_stat_item local_stat = NUMA_LOCAL;
> +	bool z_is_cpuless = !node_state(zone_to_nid(z), N_CPU);
> +	bool pref_is_cpuless = !node_state(zone_to_nid(preferred_zone), N_CPU);
>  
> -	/* skip numa counters update if numa stats is disabled */
>  	if (!static_branch_likely(&vm_numa_stat_key))
>  		return;
>  
> -	if (zone_to_nid(z) != numa_node_id())
> +	if (zone_to_nid(z) != numa_node_id() || z_is_cpuless)
>  		local_stat = NUMA_OTHER;
>  
> -	if (zone_to_nid(z) == zone_to_nid(preferred_zone))
> +	if (zone_to_nid(z) == zone_to_nid(preferred_zone) && !z_is_cpuless)
>  		__count_numa_events(z, NUMA_HIT, nr_account);
>  	else {
>  		__count_numa_events(z, NUMA_MISS, nr_account);
> -		__count_numa_events(preferred_zone, NUMA_FOREIGN, nr_account);
> +		if (!pref_is_cpuless)
> +			__count_numa_events(preferred_zone, NUMA_FOREIGN, nr_account);
>  	}
>  	__count_numa_events(z, local_stat, nr_account);
>  #endif
> _
> 
> Patches currently in -mm which might be from dongjoo.linux.dev@gmail.com are
> 
> mm-page_alloc-fix-numa-stats-update-for-cpu-less-nodes.patch

-- 
Michal Hocko
SUSE Labs