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

[Mathieu Desnoyers]

On 2025-03-26 03:31, Gabriele Monaco wrote:
> 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?

I agree. I've reviewed the entire series a few weeks ago and it
looks good to me.

Thanks,

Mathieu

> 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


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com