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Added CCs. ] On 2025-02-10 08:56, Gabriele Monaco wrote: > This patchset moves the task_mm_cid_work to a preemptible and migratable > context. This reduces the impact of this task to the scheduling latency > of real time tasks. > The change makes the recurrence of the task a bit more predictable. > We also add optimisation and fixes to make sure the task_mm_cid_work > works as intended. > > 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 allows the mm_cids to be actually compacted when a process > reduces its number of threads, which was not the case since the same > mm_cids were reused to improve cache locality, more details in [4]. > > Patch 2 contains the main changes, removing the task_work on the > scheduler tick and using a delayed_work instead. > Additionally, we terminate the call immediately if we see that no mm_cid > is actually active, which could happen on processes sleeping for long > time or which exited but whose mm has not been freed yet. > > Patch 3 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 is not applied and is flaky without patch 2. We expect it to always > pass with the entire patchset applied. There is a tiny nit in a comment in patch 3. Other than that the series looks good. Please re-send with this fixed and please add my reviewed by tag to patch 3. Once updated, Peter, Ingo, you have my blessing to pull this into tip. Thanks! Mathieu > > Changes since V4 [1]: > * Fixes on the selftest > * Polished memory allocation and cleanup > * Handle the test failure in main > > Changes since V3 [2]: > * 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 [3]: > * 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 [4]: > * 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 > > Overhead comparison in [4] > > [1] - https://lore.kernel.org/lkml/20250113074231.61638-4-gmonaco@redhat.com > [2] - https://lore.kernel.org/lkml/20241216130909.240042-1-gmonaco@redhat.com > [3] - https://lore.kernel.org/lkml/20241213095407.271357-1-gmonaco@redhat.com > [4] - https://lore.kernel.org/lkml/20241205083110.180134-2-gmonaco@redhat.com > > To: Mathieu Desnoyers > To: Peter Zijlstra > To: Ingo Molnar > > Gabriele Monaco (2): > sched: Move task_mm_cid_work to mm delayed work > rseq/selftests: Add test for mm_cid compaction > > Mathieu Desnoyers (1): > sched: Compact RSEQ concurrency IDs with reduced threads and affinity > > include/linux/mm_types.h | 23 +- > include/linux/sched.h | 1 - > kernel/sched/core.c | 66 +----- > kernel/sched/sched.h | 32 ++- > tools/testing/selftests/rseq/.gitignore | 1 + > tools/testing/selftests/rseq/Makefile | 2 +- > .../selftests/rseq/mm_cid_compaction_test.c | 200 ++++++++++++++++++ > 7 files changed, 246 insertions(+), 79 deletions(-) > create mode 100644 tools/testing/selftests/rseq/mm_cid_compaction_test.c > > > base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3 -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com