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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Mar 09, 2026 at 09:17:43PM -0700, JP Kobryn (Meta) wrote:
> On 3/9/26 4:43 PM, Shakeel Butt wrote:
> > On Fri, Mar 06, 2026 at 08:55:20PM -0800, JP Kobryn (Meta) wrote:
[...]
> > 
> > This seems like monotonic increasing metrics and I think you don't care about
> > their absolute value but rather rate of change. Any reason this can not be
> > achieved through tracepoints and BPF combination?
> 
> We have the per-node reclaim stats (pg{steal,scan,refill}) in
> nodeN/vmstat and memory.numa_stat now. The new stats in this patch would
> be collected from the same source. They were meant to be used together,
> so it seemed like a reasonable location. I think the advantage over
> tracepoints is we get the observability on from the start and it would
> be simple to extend existing programs that already read stats from the
> cgroup dir files.

Convenience is not really justifying the cost of adding 18 counters,
particularly in memcg. We can argue about adding just in system level metrics
but not for memcg.

counter_cost = nr_cpus * nr_nodes * nr_memcg * 16 (struct lruvec_stats_percpu)

On a typical prod machine, we can see 1000s of memcg, 100s of cpus and couple of
numa nodes. So, a single counter's cost can range from 200KiB to MiBs. This does
not seem like a cost we should force everyone to pay.

If you really want these per-memcg and assuming these metrics are updated in
non-performance critical path, we can try to decouple these and other reclaim
related stats from rstat infra. That would at least reduce nr_cpus factor in the
above equation to 1. Though we will need to actually evaluate the performance
for the change before committing to it.