The commit 851a723e45d1c("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
would cause that online/offline cpu will produce different results
for the !top-cpuset task.
For example:

If the task was running, then offline task's cpus, would lead to clear
its user-mask.

unisoc:/ # while true; do sleep 600; done&
[1] 6786
unisoc:/ # echo 6786 > /dev/cpuset/top-app/tasks
unisoc:/ # cat /dev/cpuset/top-app/cpus
0-7
unisoc:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   ff
Cpus_allowed_list:      0-7

unisoc:/ # taskset -p c0 6786
pid 6786's current affinity mask: ff
pid 6786's new affinity mask: c0
unisoc:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   c0
Cpus_allowed_list:      6-7

After offline the cpu6 and cpu7, the user-mask would be cleared:

unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu7/online
unisoc:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   40
Cpus_allowed_list:      6
ums9621_1h10:/ # echo 0 > /sys/devices/system/cpu/cpu6/online
ums9621_1h10:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   3f
Cpus_allowed_list:      0-5

When online the cpu6/7, the user-mask can not bring back:

unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu6/online
unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu7/online
unisoc:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   ff
Cpus_allowed_list:      0-6

However, if we offline the cpu when the task is sleeping, at this
time, because would not call the fallback_cpu(), its user-mask will
not be cleared.

unisoc:/ # while true; do sleep 600; done&
[1] 5990
unisoc:/ # echo 5990 > /dev/cpuset/top-app/tasks
unisoc:/ # cat /proc/5990/status | grep Cpus
Cpus_allowed:   ff
Cpus_allowed_list:      0-7

unisoc:/ # taskset -p c0 5990
pid 5990's current affinity mask: ff
pid 5990's new affinity mask: c0
unisoc:/ # cat /proc/5990/status | grep Cpus
Cpus_allowed:   c0
Cpus_allowed_list:      6-7

unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu6/online
unisoc:/ # cat /proc/5990/status | grep Cpus
Cpus_allowed:   80
Cpus_allowed_list:      7
unisoc:/ # echo 0 > /sys/devices/system/cpu/cpu7/online
unisoc:/ # cat /proc/5990/status | grep Cpus
Cpus_allowed:   3f
Cpus_allowed_list:      0-5

After 10 minutes, it was waked up, it can also keep its user-mask:
ums9621_1h10:/ # cat /proc/5990/status | grep Cpus
Cpus_allowed:   3f
Cpus_allowed_list:      0-5

And when online the cpu6/7,the user-mask could bring back.
unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu6/online
unisoc:/ # echo 1 > /sys/devices/system/cpu/cpu7/online
unisoc:/ # cat /proc/6786/status | grep Cpus
Cpus_allowed:   c0
Cpus_allowed_list:      6-7

Indeed, there is no need to clear the user_cpus_ptr if there is an
intersection between user_cpus_ptr and new_mask.
So add the judgement of whether there is an intersection between them.
Clear user_cpus_ptr only when no intersection with the new mask.
In this way, the above problems can also be solved.

Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
---
previous discussion:
 https://lore.kernel.org/all/e402d623-1875-47a2-9db3-8299a54502ef@redhat.com/
---
 kernel/sched/core.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7019a40457a6..bbb8e88949f4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2796,21 +2796,24 @@ __do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
 }
 
 /*
- * Used for kthread_bind() and select_fallback_rq(), in both cases the user
- * affinity (if any) should be destroyed too.
+ * Used for kthread_bind() and select_fallback_rq().
+ * Destroy user affinity if no intersection with the new_mask.
  */
 void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 {
 	struct affinity_context ac = {
 		.new_mask  = new_mask,
 		.user_mask = NULL,
-		.flags     = SCA_USER,	/* clear the user requested mask */
+		.flags     = 0,
 	};
 	union cpumask_rcuhead {
 		cpumask_t cpumask;
 		struct rcu_head rcu;
 	};
 
+	if (p->user_cpus_ptr && !cpumask_intersects(p->user_cpus_ptr, new_mask))
+		ac.flags = SCA_USER;	/* clear the user requested mask */
+
 	__do_set_cpus_allowed(p, &ac);
 
 	/*
-- 
2.25.1