[v2 PATCH 0/1] fs/proc: Expose mm_cpumask in /proc/[pid]/status

[Aaron Tomlin <atomlin@atomlin.com>]

Hi Oleg, David, Greg, Andrew,

This patch introduces two new fields to /proc/[pid]/status to display the
set of CPUs, representing the CPU affinity of the process's active memory
context, in both mask and list format: "Cpus_active_mm" and
"Cpus_active_mm_list". The mm_cpumask is primarily used for TLB and cache
synchronisation.
    
Exposing this information allows userspace to easily describe the
relationship between CPUs where a memory descriptor is "active" and the
CPUs where the thread is allowed to execute. The primary intent is to
provide visibility into the "memory footprint" across CPUs, which is
invaluable for debugging performance issues related to IPI storms and TLB
shootdowns in large-scale NUMA systems. The CPU-affinity sets the boundary;
the mm_cpumask records the arrival; they complement each other.
    
Frequent mm_cpumask changes may indicate instability in placement policies
or excessive task migration overhead.


Changes since v1:
 - Document new Cpus_active_mm and Cpus_active_mm_list entries in
   /proc/[pid]/status (Oleg Nesterov)

[1]: https://lore.kernel.org/lkml/20251217024603.1846651-1-atomlin@atomlin.com/

Aaron Tomlin (1):
  fs/proc: Expose mm_cpumask in /proc/[pid]/status

 Documentation/filesystems/proc.rst |  3 +++
 fs/proc/array.c                    | 22 +++++++++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

-- 
2.51.0