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 identify
memory-task affinity, insight to NUMA alignment, CPU isolation and
real-time workload placement.

Frequent mm_cpumask changes may indicate instability in placement
policies or excessive task migration overhead.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 fs/proc/array.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 42932f88141a..8887c5e38e51 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -409,6 +409,23 @@ static void task_cpus_allowed(struct seq_file *m, struct task_struct *task)
 		   cpumask_pr_args(&task->cpus_mask));
 }
 
+/**
+ * task_cpus_active_mm - Show the mm_cpumask for a process
+ * @m: The seq_file structure for the /proc/PID/status output
+ * @mm: The memory descriptor of the process
+ *
+ * Prints the set of CPUs, representing the CPU affinity of the process's
+ * active memory context, in both mask and list format. This mask is
+ * primarily used for TLB and cache synchronisation.
+ */
+static void task_cpus_active_mm(struct seq_file *m, struct mm_struct *mm)
+{
+	seq_printf(m, "Cpus_active_mm:\t%*pb\n",
+		   cpumask_pr_args(mm_cpumask(mm)));
+	seq_printf(m, "Cpus_active_mm_list:\t%*pbl\n",
+		   cpumask_pr_args(mm_cpumask(mm)));
+}
+
 static inline void task_core_dumping(struct seq_file *m, struct task_struct *task)
 {
 	seq_put_decimal_ull(m, "CoreDumping:\t", !!task->signal->core_state);
@@ -450,12 +467,15 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
 		task_core_dumping(m, task);
 		task_thp_status(m, mm);
 		task_untag_mask(m, mm);
-		mmput(mm);
 	}
 	task_sig(m, task);
 	task_cap(m, task);
 	task_seccomp(m, task);
 	task_cpus_allowed(m, task);
+	if (mm) {
+		task_cpus_active_mm(m, mm);
+		mmput(mm);
+	}
 	cpuset_task_status_allowed(m, task);
 	task_context_switch_counts(m, task);
 	arch_proc_pid_thread_features(m, task);
-- 
2.51.0