Re: [patch V5 00/20] sched: Rewrite MM CID management

[Thomas Gleixner <tglx@kernel.org>]

On Wed, Jan 28 2026 at 14:56, Thomas Gleixner wrote:
> On Wed, Jan 28 2026 at 18:28, Shrikanth Hegde wrote:
>> On 1/28/26 5:27 PM, Thomas Gleixner wrote:
>>   watchdog: CPU 23 self-detected hard LOCKUP @ mm_get_cid+0xe8/0x188
>>   watchdog: CPU 23 TB:1434903268401795, last heartbeat TB:1434897252302837 (11750ms ago)
>>   NIP [c0000000001b7134] mm_get_cid+0xe8/0x188
>>   LR [c0000000001b7154] mm_get_cid+0x108/0x188
>>   Call Trace:
>>   [c000000004c37db0] [c000000001145d84] cpuidle_enter_state+0xf8/0x6a4 (unreliable)
>>   [c000000004c37e00] [c0000000001b95ac] mm_cid_switch_to+0x3c4/0x52c
>>   [c000000004c37e60] [c000000001147264] __schedule+0x47c/0x700
>
> So if the above spins in mm_get_cid() then the below is just a consequence.
>
>>   watchdog: CPU 11 self-detected hard LOCKUP @ plpar_hcall_norets_notrace+0x18/0x2c
>>   watchdog: CPU 11 TB:1434903340004919, last heartbeat TB:1434897249749892 (11895ms ago)
>>   NIP [c0000000000f84fc] plpar_hcall_norets_notrace+0x18/0x2c
>>   LR [c000000001152588] queued_spin_lock_slowpath+0xd88/0x15d0
>>   Call Trace:
>>   [c00000056b69fb10] [c00000056b69fba0] 0xc00000056b69fba0 (unreliable)
>>   [c00000056b69fc30] [c000000001153ce0] _raw_spin_lock+0x80/0xa0
>>   [c00000056b69fc50] [c0000000001b9a34] raw_spin_rq_lock_nested+0x3c/0xf8
>>   [c00000056b69fc80] [c0000000001b9bb8] mm_cid_fixup_cpus_to_tasks+0xc8/0x28c
>>   [c00000056b69fd00] [c0000000001bff34] sched_mm_cid_exit+0x108/0x22c
>>   [c00000056b69fd40] [c000000000167b08] do_exit+0xf4/0x5d0
>>   [c00000056b69fdf0] [c00000000016800c] make_task_dead+0x0/0x178
>>   [c00000056b69fe10] [c0000000000316c8] system_call_exception+0x128/0x390
>>   [c00000056b69fe50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec
>
>> I am wondering if it this loop in mm_get_cid, which may not be getting a cid
>> for a long time? Is that possible?
>
> It shouldn't be possible by design, but it seems there is a corner case
> lurking somewhere which hasn't been covered. Let me stare at the logic
> in the transition functions once more. That's where CPU11 comes from:
>
>>   [c00000056b69fc80] [c0000000001b9bb8] mm_cid_fixup_cpus_to_tasks+0xc8/0x28c
>
> The exiting it initiated a transition back from per CPU to per task mode
> and that seems to make things unhappy for mysterious reasons.

I stared at it for a while and found the below stupidity. But when I
actually sat down after a while away from the keyboard and tried to
write a concise changelog explaining the root cause I failed to come up
with a coherent explanation why this would prevent the above scenario,
which hints at a situation of MMCID exhaustion.

@Ihor: Is the BPF CI fallout reproducible? If so, can you please provide
       it?

Thanks,

        tglx
---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10664,8 +10664,14 @@ void sched_mm_cid_exit(struct task_struc
 			scoped_guard(raw_spinlock_irq, &mm->mm_cid.lock) {
 				if (!__sched_mm_cid_exit(t))
 					return;
-				/* Mode change required. Transfer currents CID */
-				mm_cid_transit_to_task(current, this_cpu_ptr(mm->mm_cid.pcpu));
+				/*
+				 * Mode change. The task has the CID unset
+				 * already. The CPU CID is still valid and
+				 * does not have MM_CID_TRANSIT set as the
+				 * mode change has just taken effect under
+				 * mm::mm_cid::lock. Drop it.
+				 */
+				mm_drop_cid_on_cpu(mm, this_cpu_ptr(mm->mm_cid.pcpu));
 			}
 			mm_cid_fixup_cpus_to_tasks(mm);
 			return;