Re: [PATCH v2] mm/mempolicy: track page allocations per mempolicy

["JP Kobryn (Meta)" <jp.kobryn@linux.dev>]

On 3/12/26 6:40 AM, Vlastimil Babka (SUSE) wrote:
> On 3/7/26 05:55, JP Kobryn (Meta) wrote:
>> When investigating pressure on a NUMA node, there is no straightforward way
>> to determine which policies are driving allocations to it.
>>
>> Add per-policy page allocation counters as new node stat items. These
>> counters track allocations to nodes and also whether the allocations were
>> intentional or fallbacks.
>>
>> The new stats follow the existing numa hit/miss/foreign style and have the
>> following meanings:
>>
>>    hit
>>      - for BIND and PREFERRED_MANY, allocation succeeded on node in nodemask
>>      - for other policies, allocation succeeded on intended node
>>      - counted on the node of the allocation
>>    miss
>>      - allocation intended for other node, but happened on this one
>>      - counted on other node
>>    foreign
>>      - allocation intended on this node, but happened on other node
>>      - counted on this node
>>
>> Counters are exposed per-memcg, per-node in memory.numa_stat and globally
>> in /proc/vmstat.
>>
>> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
> 
> I think I've been on of the folks on previous versions arguing against the
> many counters, and one of the arguments was it they can't tell the full
> story anyway (compared to e.g. tracing), but I don't think adding even more
> counters is the right solution. Seems like a number of other people
> responding to the thread are providing similar feedback.
> 
> For example I'm still not sure how it would help me if I knew the
> hits/misses were due to a preferred vs preferred_many policy, or interleave
> vs weithed interleave?
> 

How about I change from per-policy hit/miss/foreign triplets to a single
aggregated policy triplet (i.e. just 3 new counters which account for
all policies)? They would follow the same hit/miss/foreign semantics
already proposed (visible in quoted text above). This would still
provide the otherwise missing signal of whether policy-driven
allocations to a node are intentional or fallback.

Note that I am also planning on moving the stats off of the memcg so the
3 new counters will be global per-node in response to similar feedback.