linux-kernel - Re: [patch 0/4] sched/mmcid: Cure mode transition woes

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Message-ID: <969dda1a-1126-4313-80dc-b2df8e5435e1@linux.dev>
Date: Thu, 29 Jan 2026 16:20:04 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Thomas Gleixner <tglx@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Jeanson <mjeanson@...icios.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [patch 0/4] sched/mmcid: Cure mode transition woes

On 1/29/26 1:20 PM, Thomas Gleixner wrote:
> Ihor and Shrikanth reported hard lockups which can be tracked back to the recent
> rewrite of the MM_CID management code.
> 
>   1) The from task to CPU ownership transition lacks the intermediate
>      transition mode, which can lead to CID pool exhaustion and a
>      subsequent live lock. That intermediate mode was implemented for the
>      reverse operation already but omitted for this transition as the
>      original analysis missed a few possible scheduling scenarios.
> 
>   2) Weakly ordered architectures can observe inconsistent state which
>      causes them to make the wrong decision. That leads to the same problem
>      as with #1.
> 
> The following series addresses these issue and fixes another albeit harmless
> inconsistent state hickup which was found when analysing the above issues.

Thomas, thank you for addressing this so quickly.

I applied the series as temporary BPF CI specific patches.
I'll report if I notice any relevant issues.

> 
> With these issues addressed the last change optimizes the bitmap
> utilization in the transition modes.
> 
> The series applies on Linus tree and passes the selftests and a thread pool
> emulator which stress tests the ownership transitions.
> 
> Thanks,
> 
> 	tglx
> ---
>  include/linux/rseq_types.h |    7 -
>  kernel/sched/core.c        |  170 +++++++++++++++++++++++++++++----------------
>  kernel/sched/sched.h       |   45 +++++++++--
>  3 files changed, 151 insertions(+), 71 deletions(-)