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McKenney" , Shuah Khan Cc: Gabriele Monaco Subject: [PATCH v7 0/2] sched: Restructure task_mm_cid_work for predictability Date: Wed, 19 Feb 2025 12:31:06 +0100 Message-ID: <20250219113108.325545-1-gmonaco@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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 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 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 2 adds a selftest to validate the functionality of the task_mm_cid_work (i.e. to compact the mm_cids). The test fails if patch 1 of V6 ("sched: Compact RSEQ concurrency IDs with reduced threads and affinity", applied separately) is not applied and is flaky without patch 1 of this series. We expect it to always pass with both applied. Changes since V6 [1]: * 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 [2]: * Fixes on the selftest * Polished memory allocation and cleanup * Handle the test failure in main Changes since V3 [3]: * 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 [4]: * 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 [5]: * 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/20250210153253.460471-1-gmonaco@redhat.com [2] - https://lore.kernel.org/lkml/20250113074231.61638-4-gmonaco@redhat.com [3] - https://lore.kernel.org/lkml/20241216130909.240042-1-gmonaco@redhat.com [4] - https://lore.kernel.org/lkml/20241213095407.271357-1-gmonaco@redhat.com [5] - https://lore.kernel.org/lkml/20241205083110.180134-2-gmonaco@redhat.com To: Mathieu Desnoyers To: Peter Zijlstra To: Ingo Molnar To: Paul E. McKenney To: Shuah Khan Gabriele Monaco (2): sched: Move task_mm_cid_work to mm work_struct selftests/rseq: Add test for mm_cid compaction include/linux/mm_types.h | 8 + include/linux/sched.h | 7 +- kernel/rseq.c | 1 + kernel/sched/core.c | 33 ++- 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 ++++++++++++++++++ 8 files changed, 229 insertions(+), 25 deletions(-) create mode 100644 tools/testing/selftests/rseq/mm_cid_compaction_test.c base-commit: 0ad2507d5d93f39619fc42372c347d6006b64319 prerequisite-patch-id: 6888a889ab5dcd50c3874005018576fdced3a5e5 -- 2.48.1