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

Shrikanth Hegde
2025-10-30T05:01:10Z

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@linutronix.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