Re: [tip: sched/core] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Peter.

On 11/14/25 5:49 PM, tip-bot2 for Tim Chen wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     2265c5d4deeff3bfe4580d9ffe718fd80a414cac
> Gitweb:        https://git.kernel.org/tip/2265c5d4deeff3bfe4580d9ffe718fd80a414cac
> Author:        Tim Chen <tim.c.chen@linux.intel.com>
> AuthorDate:    Mon, 10 Nov 2025 10:47:35 -08:00
> Committer:     Peter Zijlstra <peterz@infradead.org>
> CommitterDate: Fri, 14 Nov 2025 13:03:05 +01:00
> 
> sched/fair: Skip sched_balance_running cmpxchg when balance is not due
> 
> 

   
> +	if (!need_unlock && (sd->flags & SD_SERIALIZE) && idle != CPU_NEWLY_IDLE) {
> +		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))

This should be atomic_cmpxchg_acquire?

I booted the system with latest sched/core and it crashes at the boot.

BUG: Kernel NULL pointer dereference on read at 0x00000000
Faulting instruction address: 0xc0000000001db57c
Oops: Kernel access of bad area, sig: 7 [#1]
LE PAGE_SIZE=64K MMU=Radix  SMP NR_CPUS=8192 NUMA pSeries
Modules linked in:
CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.18.0-rc3+ #242 PREEMPT(lazy)
NIP [c0000000001db57c] sched_balance_rq+0x560/0x92c
LR [c0000000001db198] sched_balance_rq+0x17c/0x92c
Call Trace:
[c00000111ffdfd10] [c0000000001db198] sched_balance_rq+0x17c/0x92c (unreliable)
[c00000111ffdfe50] [c0000000001dc598] sched_balance_domains+0x2c4/0x3d0
[c00000111ffdff00] [c000000000168958] handle_softirqs+0x138/0x414
[c00000111ffdffe0] [c000000000017d80] do_softirq_own_stack+0x3c/0x50
[c000000008a57a60] [c000000000168048] __irq_exit_rcu+0x18c/0x1b4
[c000000008a57a90] [c0000000001691a8] irq_exit+0x20/0x38
[c000000008a57ab0] [c000000000028c18] timer_interrupt+0x174/0x394
[c000000008a57b10] [c000000000009f8c] decrementer_common_virt+0x28c/0x290


Bisect pointed to:
git bisect bad 2265c5d4deeff3bfe4580d9ffe718fd80a414cac
# first bad commit: [2265c5d4deeff3bfe4580d9ffe718fd80a414cac] sched/fair: Skip sched_balance_running cmpxchg when balance is not due


I wondered what is really different since the tim's v4 boots fine.
There is try instead in the tip, i think that is messing it since likely
we are dereferencing 0?


With this diff it boots fine.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index aaa47ece6a8e..01814b10b833 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11841,7 +11841,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
         }
  
         if (!need_unlock && (sd->flags & SD_SERIALIZE)) {
-               if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
+               if (!atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
                         goto out_balanced;
  
                 need_unlock = true;