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

[Tim Chen]

On Sun, 2025-11-16 at 02:26 +0530, Shrikanth Hegde wrote:
> 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@xxxxxxxxxxxxxxx>
> > AuthorDate:    Mon, 10 Nov 2025 10:47:35 -08:00
> > Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > 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))

The second argument of atomic_try_cmpxchg_acquire is "int *old" while that of atomic_cmpxchg_acquire
is "int old". So the above check would result in NULL pointer access.  Probably have
to do something like the following to use atomic_try_cmpxchg_acquire()

		int zero = 0;
		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, &zero, 1))
		
Otherwise we should do atomic_cmpxchg_acquire() as below

> +               if (!atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))

Tim

>                          goto out_balanced;
>   
>                  need_unlock = true;
> 
>