linux-kernel - Re: [patch V3 00/20] sched: Rewrite MM CID management

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Message-ID: <47e73026-390e-40d6-a860-10e9378a3bf3@linux.ibm.com>
Date: Thu, 30 Oct 2025 10:30:24 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
        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>
Subject: Re: [patch V3 00/20] sched: Rewrite MM CID management

Hi Thomas.

On 10/29/25 6:38 PM, Thomas Gleixner wrote:
> This is a follow up on V2 series which can be found here:
> 
>      https://lore.kernel.org/20251022104005.907410538@linutronix.de
> 
> The V1 cover letter contains a detailed analyisis of the issues:
> 
>      https://lore.kernel.org/20251015164952.694882104@linutronix.de
> 
> TLDR: The CID management is way to complex and adds significant overhead
> into scheduler hotpaths.
> 
> The series rewrites MM CID management in a more simplistic way which
> focusses on low overhead in the scheduler while maintaining per task CIDs
> as long as the number of threads is not exceeding the number of possible
> CPUs.
> 
> The series is based on the V6 series of the rseq rewrite:
> 
>      https://lore.kernel.org/20251027084220.785525188@linutronix.de
> 
> which is also available from git:
> 
>      git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core/rseq
> 
> The series on top of the tip core/rseq branch is available from git as
> well:
> 
>      git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git rseq/cid
> 
> Changes vs. V2:
> 
>     - Rename to cpumask/bitmap_weighted_or() - Yury
> 
>     - Zero the bitmap with length of bitmap_size(nr_possible_cpus()) -
>       Shrikanth
>     
>     - Move cpu_relax() out of for() as that fails to build when cpu_relax()
>       is a macro. - Shrikanth
> 
>     - Picked up Reviewed/Acked-by tags where appropriate
> 
> Thanks,
> 
> 	tglx
> ---
> Thomas Gleixner (20):
>        sched/mmcid: Revert the complex CID management
>        sched/mmcid: Use proper data structures
>        sched/mmcid: Cacheline align MM CID storage
>        sched: Fixup whitespace damage
>        sched/mmcid: Move scheduler code out of global header
>        sched/mmcid: Prevent pointless work in mm_update_cpus_allowed()
>        cpumask: Introduce cpumask_weighted_or()
>        sched/mmcid: Use cpumask_weighted_or()
>        cpumask: Cache num_possible_cpus()
>        sched/mmcid: Convert mm CID mask to a bitmap
>        signal: Move MMCID exit out of sighand lock
>        sched/mmcid: Move initialization out of line
>        sched/mmcid: Provide precomputed maximal value
>        sched/mmcid: Serialize sched_mm_cid_fork()/exit() with a mutex
>        sched/mmcid: Introduce per task/CPU ownership infrastrcuture
>        sched/mmcid: Provide new scheduler CID mechanism
>        sched/mmcid: Provide CID ownership mode fixup functions
>        irqwork: Move data struct to a types header
>        sched/mmcid: Implement deferred mode change
>        sched/mmcid: Switch over to the new mechanism
> 
>   include/linux/bitmap.h         |   15
>   include/linux/cpumask.h        |   26 +
>   include/linux/irq_work.h       |    9
>   include/linux/irq_work_types.h |   14
>   include/linux/mm_types.h       |  125 ------
>   include/linux/rseq.h           |   27 -
>   include/linux/rseq_types.h     |   71 +++
>   include/linux/sched.h          |   19
>   init/init_task.c               |    3
>   kernel/cpu.c                   |   15
>   kernel/exit.c                  |    1
>   kernel/fork.c                  |    7
>   kernel/sched/core.c            |  815 +++++++++++++++++++----------------------
>   kernel/sched/sched.h           |  395 ++++++++-----------
>   kernel/signal.c                |    2
>   lib/bitmap.c                   |    6
>   16 files changed, 727 insertions(+), 823 deletions(-)
> 
> 

I am running into crash at boot on power10 pseries.
Thought of putting it here first. Me trying to figure out why.

I am using your tree.
git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git

commit 789ff6e7cc5aa423473eb135f94812fe77b8aeab (HEAD -> rseq/cid, origin/rseq/cid)
Author: Thomas Gleixner <tglx@...utronix.de>
Date:   Tue Oct 14 10:51:04 2025 +0200

     sched/mmcid: Switch over to the new mechanism


Oops: Kernel access of bad area, sig: 7 [#3]
LE PAGE_SIZE=64K MMU=Radix  SMP NR_CPUS=8192 NUMA pSeries
Modules linked in: drm drm_panel_orientation_quirks xfs sd_mod sg ibmvscsi ibmveth scsi_transport_srp pseries_wdt dm_mirror dm_region_hash dm_log dm_mod fuse
CPU: 96 UID: 0 PID: 0 Comm: swapper/96 Tainted: G      D W           6.18.0-rc3+ #4 PREEMPT(lazy)
Tainted: [D]=DIE, [W]=WARN
NIP [c0000000001b5c10] mm_cid_switch_to+0x58/0x52c
LR [c000000001117c84] __schedule+0x4bc/0x760
Call Trace:
[c00000668367fde0] [c0000000001b53c8] __pick_next_task+0x60/0x2ac (unreliable)
[c00000668367fe40] [c000000001117a14] __schedule+0x24c/0x760
[c00000668367fee0] [c0000000011183d0] schedule_idle+0x3c/0x64
[c00000668367ff10] [c0000000001f2470] do_idle+0x15c/0x1ac
[c00000668367ff60] [c0000000001f2788] cpu_startup_entry+0x4c/0x50
[c00000668367ff90] [c00000000005ef20] start_secondary+0x284/0x288
[c00000668367ffe0] [c00000000000e158] start_secondary_prolog+0x10/0x14