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

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Message-ID: <87y0lh96xo.ffs@tglx>
Date: Wed, 28 Jan 2026 14:56:51 +0100
From: Thomas Gleixner <tglx@...nel.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, Peter Zijlstra
 <peterz@...radead.org>, Ihor Solodrai <ihor.solodrai@...ux.dev>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Gabriele Monaco <gmonaco@...hat.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Michael Jeanson <mjeanson@...icios.com>,
 Jens Axboe <axboe@...nel.dk>, "Paul E. McKenney" <paulmck@...nel.org>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>, Florian Weimer
 <fweimer@...hat.com>, Tim Chen <tim.c.chen@...el.com>, Yury Norov
 <yury.norov@...il.com>, bpf <bpf@...r.kernel.org>,
 sched-ext@...ts.linux.dev, Kernel Team <kernel-team@...a.com>, Alexei
 Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Puranjay Mohan <puranjay@...nel.org>,
 Tejun Heo <tj@...nel.org>
Subject: Re: [patch V5 00/20] sched: Rewrite MM CID management

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