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

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

On 3/12/26 10:07 PM, Huang, Ying wrote:
> "JP Kobryn (Meta)" <jp.kobryn@linux.dev> writes:
> 
>> 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.
> 
> Emm, what's the difference between these newly added counters and the
> existing numa_hit/miss/foreign counters?

The existing counters don't account for node masks in the policies that
make use of them. An allocation can land on a node in the mask and still
be considered a miss because it wasn't the preferred node.