Re: [PATCH v2] sched/cache: Refresh LLC capacity across CPU hotplug

[K Prateek Nayak]

Hello Davi,

On 9/12/2026 3:32 AM, Davi Chaves Azevedo wrote:
> The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
> covered by a domain:
> 
>   llc_bytes = cache_size * span_weight / shared_weight
> 
> During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
> before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
> new domains therefore use the old sharing weight. The later call to
> sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
> already been detached, and returns without correcting the surviving CPUs.
> 
> On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
> offlining one SMT sibling left the remaining CPUs with:
> 
>   llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes
> 
> The correct capacity is still 16777216 bytes. On systems with active
> cache-aware scheduling, an underestimated capacity can cause
> exceed_llc_capacity() to reject aggregation for a process whose footprint
> would fit. Unchanged cpuset partitions sharing the physical cache can
> also retain stale capacity when a CPU comes online in another partition.
> 
> Pass the cache-sharing mask already retained by cacheinfo to the
> scheduler update. Refresh every surviving CPU using its own LLC domain
> so that each partition receives the correct share. This also preserves
> the correction needed as cache-sharing maps grow during boot.
> 
> Keep the existing CPU-hotplug and scheduler-domain synchronization. The
> update remains on the hotplug path; no steady-state scheduling operation
> or persistent allocation is added.
> 
> Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
> Signed-off-by: Davi Chaves Azevedo <davichazbh@xxxxxxxxx>
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

-- 
Thanks and Regards,
Prateek