linux-kernel - [PATCH v2 2/2] fs/proc: Show the content of task->user_cpus

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Message-ID: <20251029215555.1006595-3-longman@redhat.com>
Date: Wed, 29 Oct 2025 17:55:55 -0400
From: Waiman Long <longman@...hat.com>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Nico Pache <npache@...hat.com>,
	Phil Auld <pauld@...hat.com>,
	John Coleman <jocolema@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH v2 2/2] fs/proc: Show the content of task->user_cpus_ptr in /proc/<pid>/status

The task->user_cpus_ptr was introduced by commit b90ca8badbd1 ("sched:
Introduce task_struct::user_cpus_ptr to track requested affinity") to
keep track of user-requested CPU affinity. With commit da019032819a
("sched: Enforce user requested affinity"), user_cpus_ptr will
persistently affect how cpus_allowed will be set.

As child processes inherit the user_cpus_ptr setting from its parent,
they may not know that their user_cpus_ptr may be set changing their
CPU affinity in an unexpected way. Currently there is not an easy way
to figure out if their user_cpus_ptr is set even if they have never
called sched_setaffinity(2) themselves. So it makes sense to enable
users to see the presence of a previously set user_cpus_ptr so they
can do something about it without getting a surprise.

Add new "Cpus_user" and "Cpus_user_list" fields to /proc/<pid>/status
output via task_cpus_allowed() as the presence of user_cpus_ptr will
affect their cpus_allowed cpumask. The new fields will be empty if
user_cpus_ptr isn't set.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 Documentation/filesystems/proc.rst | 3 +++
 fs/proc/array.c                    | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 0b86a8022fa1..4317c79a530f 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -291,6 +291,9 @@ It's slow but very precise.
  SpeculationIndirectBranch   indirect branch speculation mode
  Cpus_allowed                mask of CPUs on which this process may run
  Cpus_allowed_list           Same as previous, but in "list format"
+ Cpus_user                   mask of user requested CPUs from
+                             sched_setaffinity(2), empty if not defined
+ Cpus_user_list              Same as previous, but in "list format"
  Mems_allowed                mask of memory nodes allowed to this process
  Mems_allowed_list           Same as previous, but in "list format"
  voluntary_ctxt_switches     number of voluntary context switches
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 2ae63189091e..17e700556daa 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -403,10 +403,19 @@ static inline void task_context_switch_counts(struct seq_file *m,
 
 static void task_cpus_allowed(struct seq_file *m, struct task_struct *task)
 {
+	cpumask_t *user_cpus = task->user_cpus_ptr;
+
 	seq_printf(m, "Cpus_allowed:\t%*pb\n",
 		   cpumask_pr_args(&task->cpus_mask));
 	seq_printf(m, "Cpus_allowed_list:\t%*pbl\n",
 		   cpumask_pr_args(&task->cpus_mask));
+
+	if (user_cpus) {
+		seq_printf(m, "Cpus_user:\t%*pb\n", cpumask_pr_args(user_cpus));
+		seq_printf(m, "Cpus_user_list:\t%*pbl\n", cpumask_pr_args(user_cpus));
+	} else {
+		seq_puts(m, "Cpus_user:\nCpus_user_list:\n");
+	}
 }
 
 static inline void task_core_dumping(struct seq_file *m, struct task_struct *task)
-- 
2.51.0