Re: [PATCH v12 0/3] sched: Restructure task_mm_cid_work for predictability

[Gabriele Monaco]

On Tue, 2025-03-11 at 07:28 +0100, Gabriele Monaco wrote:
> This patchset moves the task_mm_cid_work to a preemptible and
> migratable
> context. This reduces the impact of this work to the scheduling
> latency
> of real time tasks.
> The change makes the recurrence of the task a bit more predictable.
> 

The series was review and, in my opinion, is ready for inclusion.
Peter, Ingo, can we merge it?

Thanks,
Gabriele

> The behaviour causing latency was introduced in commit 223baf9d17f2
> ("sched: Fix performance regression introduced by mm_cid") which
> introduced a task work tied to the scheduler tick.
> That approach presents two possible issues:
> * the task work runs before returning to user and causes, in fact, a
>   scheduling latency (with order of magnitude significant in
> PREEMPT_RT)
> * periodic tasks with short runtime are less likely to run during the
>   tick, hence they might not run the task work at all
> 
> Patch 1 add support for prev_sum_exec_runtime to the RT, deadline and
> sched_ext classes as it is supported by fair, this is required to
> avoid
> calling rseq_preempt on tick if the runtime is below a threshold.
> 
> Patch 2 contains the main changes, removing the task_work on the
> scheduler tick and using a work_struct scheduled more reliably during
> __rseq_handle_notify_resume.
> 
> Patch 3 adds a selftest to validate the functionality of the
> task_mm_cid_work (i.e. to compact the mm_cids).
> 
> Changes since V11:
> * Remove variable to make mm_cid_needs_scan more compact
> * All patches reviewed
> 
> Changes since V10:
> * Fix compilation errors with RSEQ and/or MM_CID disabled
> 
> Changes since V9:
> * Simplify and move checks from task_queue_mm_cid to its call site
> 
> Changes since V8 [1]:
> * Add support for prev_sum_exec_runtime to RT, deadline and sched_ext
> * Avoid rseq_preempt on ticks unless executing for more than 100ms
> * Queue the work on the unbound workqueue
> 
> Changes since V7:
> * Schedule mm_cid compaction and update at every tick too
> * mmgrab before scheduling the work
> 
> Changes since V6 [2]:
> * Switch to a simple work_struct instead of a delayed work
> * Schedule the work_struct in __rseq_handle_notify_resume
> * Asynchronously disable the work but make sure mm is there while we
> run
> * Remove first patch as merged independently
> * Fix commit tag for test
> 
> Changes since V5:
> * Punctuation
> 
> Changes since V4 [3]:
> * Fixes on the selftest
>     * Polished memory allocation and cleanup
>     * Handle the test failure in main
> 
> Changes since V3 [4]:
> * Fixes on the selftest
>     * Minor style issues in comments and indentation
>     * Use of perror where possible
>     * Add a barrier to align threads execution
>     * Improve test failure and error handling
> 
> Changes since V2 [5]:
> * Change the order of the patches
> * Merge patches changing the main delayed_work logic
> * Improved self-test to spawn 1 less thread and use the main one
> instead
> 
> Changes since V1 [6]:
> * Re-arm the delayed_work at each invocation
> * Cancel the work synchronously at mmdrop
> * Remove next scan fields and completely rely on the delayed_work
> * Shrink mm_cid allocation with nr thread/affinity (Mathieu
> Desnoyers)
> * Add self test
> 
> [1] -
> https://lore.kernel.org/lkml/20250220102639.141314-1-gmonaco@xxxxxxxxxx
> [2] -
> https://lore.kernel.org/lkml/20250210153253.460471-1-gmonaco@xxxxxxxxxx
> [3] -
> https://lore.kernel.org/lkml/20250113074231.61638-4-gmonaco@xxxxxxxxxx
> [4] -
> https://lore.kernel.org/lkml/20241216130909.240042-1-gmonaco@xxxxxxxxxx
> [5] -
> https://lore.kernel.org/lkml/20241213095407.271357-1-gmonaco@xxxxxxxxxx
> [6] -
> https://lore.kernel.org/lkml/20241205083110.180134-2-gmonaco@xxxxxxxxxx
> 
> To: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> To: Ingo Molnar <mingo@xxxxxxxxxx>
> To: Paul E. McKenney <paulmck@xxxxxxxxxx>
> To: Shuah Khan <shuah@xxxxxxxxxx>
> 
> Gabriele Monaco (3):
>   sched: Add prev_sum_exec_runtime support for RT, DL and SCX classes
>   sched: Move task_mm_cid_work to mm work_struct
>   selftests/rseq: Add test for mm_cid compaction
> 
>  include/linux/mm_types.h                      |  17 ++
>  include/linux/rseq.h                          |  13 ++
>  include/linux/sched.h                         |   7 +-
>  kernel/rseq.c                                 |   2 +
>  kernel/sched/core.c                           |  43 ++--
>  kernel/sched/deadline.c                       |   1 +
>  kernel/sched/ext.c                            |   1 +
>  kernel/sched/rt.c                             |   1 +
>  kernel/sched/sched.h                          |   2 -
>  tools/testing/selftests/rseq/.gitignore       |   1 +
>  tools/testing/selftests/rseq/Makefile         |   2 +-
>  .../selftests/rseq/mm_cid_compaction_test.c   | 200
> ++++++++++++++++++
>  12 files changed, 258 insertions(+), 32 deletions(-)
>  create mode 100644
> tools/testing/selftests/rseq/mm_cid_compaction_test.c
> 
> 
> base-commit: 80e54e84911a923c40d7bee33a34c1b4be148d7a