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

[Thomas Gleixner <tglx@kernel.org>]

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.

Thanks,

        tglx