Re: [PATCH v3] sched/numa: add statistics of numa balance task migration

Hi Venkat,

On 5/6/2025 1:25 AM, Venkat Rao Bagalkote wrote:

On 05/05/25 8:33 pm, Chen, Yu C wrote:

quoted

On 5/5/2025 2:43 PM, Jain, Ayush wrote:

quoted

Hello,

Hitting Kernel Panic on latest-next while running rcutorture tests

37ff6e9a2ce3 ("Add linux-next specific files for 20250502")

reverting this patch fixes it
3b2339eeb032 ("sched-numa-add-statistics-of-numa-balance-task- 
migration-v3")
https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux- 
next.git/commit/kernel/sched/core.c? 
id=3b2339eeb032627e9329daf70a4ba8cd62c9cc8d

by looking at RIP pointer

$ ./scripts/faddr2line vmlinux __migrate_swap_task+0x2e/0x180
__migrate_swap_task+0x2e/0x180:
count_memcg_events_mm at include/linux/memcontrol.h:987
(inlined by) count_memcg_events_mm at include/linux/memcontrol.h:978
(inlined by) __migrate_swap_task at kernel/sched/core.c:3356

memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
mm->owner -> NULL

Attaching kernel logs below:

[ 1070.635450] rcu-torture: rcu_torture_read_exit: End of episode
[ 1074.047617] BUG: kernel NULL pointer dereference, address:
0000000000000498

Thanks Ayush,

According to this address,
   4c 8b af 50 09 00 00    mov    0x950(%rdi),%r13  <--- r13 = p->mm;
   49 8b bd 98 04 00 00    mov    0x498(%r13),%rdi  <--- p->mm->owner
It seems that this task to be swapped has NULL mm_struct.

Does the following help?

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 96db6947bc92..0cb8cc4d551d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -3353,7 +3353,8 @@ void set_task_cpu(struct task_struct *p, 

unsigned int new_cpu)
 static void __migrate_swap_task(struct task_struct *p, int cpu)
 {
        __schedstat_inc(p->stats.numa_task_swapped);
-       count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
+       if (p->mm)
+               count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);

        if (task_on_rq_queued(p)) {
                struct rq *src_rq, *dst_rq;

Hello Chenyu,


This issue is reported even on IBM Power servers.

Proposed fix works fine. Hence,


Tested-by: Venkat Rao Bagalkote <redacted>

Thank you for the test. May I know if you could help
check if v4 works without any NULL pointer exception?

https://lore.kernel.org/lkml/cover.1746611892.git.yu.c.chen@intel.com/ (local)

thanks,
Chenyu