Re: [Patch v4 17/22] sched/cache: Avoid cache-aware scheduling for memory-heavy processes

[Peter Zijlstra]

On Wed, Apr 01, 2026 at 02:52:29PM -0700, Tim Chen wrote:
> From: Chen Yu <yu.c.chen@xxxxxxxxx>
> 
> Prateek and Tingyin reported that memory-intensive workloads (such as
> stream) can saturate memory bandwidth and caches on the preferred LLC
> when sched_cache aggregates too many threads.
> 
> To mitigate this, estimate a process's memory footprint by comparing
> its RSS (anonymous and shared pages) to the size of the LLC. If RSS
> exceeds the LLC size, skip cache-aware scheduling.
> 
> Note that RSS is only an approximation of the memory footprint.
> By default, the comparison is strict, but a later patch will allow
> users to provide a hint to adjust this threshold.
> 
> According to the test from Adam, some systems do not have shared L3
> but with shared L2 as clusters. In this case, the L2 becomes the LLC[1].

This is pretty terrible. If you want LLC size, add it to the topology
information (and ideally integrate with RDT) and make proportional to
cpumask size, such that if someone cuts the domain in pieces, they get
proportional size etc.

Also, if we have NUMA_BALANCING on, that can provide a much better
estimate for the actual size.

Just using RSS seems like a very bad metric here.