[PATCH] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity()

[Waiman Long <longman@redhat.com>]

Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
cpumask"), user provided CPU affinity via sched_setaffinity(2) is
perserved even if the task is being moved to a different cpuset.
However, that affinity is also being inherited by any subsequently
created child processes which may not want or be aware of that affinity.

One way to solve this problem is to provide a way to back off from
that user provided CPU affinity.  This patch implements such a scheme
by using a full cpumask (a cpumask with all bits set) to signal the
clearing of the user cpumask to follow the default as allowed by
the current cpuset.  In fact, with a full cpumask in user_cpus_ptr,
the task behavior should be the same as with a NULL user_cpus_ptr.
This patch just formalizes it without causing any incompatibility and
discard an otherwise useless cpumask.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/syscalls.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 0470bcc3d204..232704f4a315 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1276,14 +1276,22 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
 		return retval;
 
 	/*
-	 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
-	 * alloc_user_cpus_ptr() returns NULL.
+	 * If a full cpumask is passed in, clear user_cpus_ptr and reset the
+	 * current cpu affinity to the default for the current cpuset.
 	 */
-	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
-	if (user_mask) {
-		cpumask_copy(user_mask, in_mask);
-	} else if (IS_ENABLED(CONFIG_SMP)) {
-		return -ENOMEM;
+	if (cpumask_full(in_mask)) {
+		user_mask = NULL;
+	} else {
+		/*
+		 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
+		 * alloc_user_cpus_ptr() returns NULL.
+		 */
+		user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
+		if (user_mask) {
+			cpumask_copy(user_mask, in_mask);
+		} else if (IS_ENABLED(CONFIG_SMP)) {
+			return -ENOMEM;
+		}
 	}
 
 	ac = (struct affinity_context){
-- 
2.47.0