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

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Message-ID: <20260129210219.452851594@kernel.org>
Date: Thu, 29 Jan 2026 22:20:46 +0100
From: Thomas Gleixner <tglx@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Ihor Solodrai <ihor.solodrai@...ux.dev>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Jeanson <mjeanson@...icios.com>
Subject: [patch 0/4] sched/mmcid: Cure mode transition woes

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.

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(-)