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. Patch 1 contains the main changes, removing the task_work on the scheduler tick and using a delayed_work instead. Patch 2 adds some optimisations to the approach, since we rely on a delayed_work, it is no longer required to check that the minimum interval passed since execution, we however 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 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 [1]. Patch 4 adds a selftest to validate the functionality of the task_mm_cid_work (i.e. to compact the mm_cids), this test requires patch 3 to be applied. Changes since V1 [1]: * 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 this section, I'm going to refer to head as the current state upstream without my patch applied, patch is the same head with these patches applied. Likewise, I'm going to refer to task_mm_cid_work as either the task or the function. The experiments are run on an aarch64 machine with 128 cores. The kernel has a bare configuration with PREEMPT_RT enabled. - Memory The patch introduces some memory overhead: * head uses a callback_head per thread (16 bytes) * patch relies on a delayed work per mm but drops a long (80 bytes net) Tasks with 5 threads or less have lower memory footprint with the current approach. Considering a task_struct can be 7-13 kB and an mm_struct is about 1.4 kB, the overhead should be acceptable. - Boot time I tested the patch booting a virtual machine with vng[2], both head and patch get similar boot times (around 8s). - Runtime I run some rather demanding tests to show what could possibly be a worst case in the approach introduced by this patch. The following tests are running again in vng to have a plain system, running mostly the stressors (if there). Unless differently specified, time is in us. All tests run for 30s. The stress-ng tests were run with 128 stressors, I will omit from the table for clarity. No load head patch running processes(threads): 12(12) 12(12) duration(avg,max,sum): 75,426,987 2,42,45ms invocations: 13 20k stress-ng --cpu-load 80 head patch running processes(threads): 129(129) 129(129) duration(avg,max,sum): 20,2ms,740ms 7,774,280ms invocations: 36k 39k stress-ng --fork head patch running processes(threads): 3.6k(3.6k) 4k(4k) duration(avg,max,sum): 34,41,720 19,457,880ms invocations: 21 46k stress-ng --pthread-max 4 head patch running processes(threads): 129(4k) 129(4k) duration(avg,max,sum): 31,195,41ms 21,1ms,830ms invocations: 1290 38k It is important to note that some of those stressors run for a very short period of time to just fork/create a thread, this heavily favours head since the task won't simply run as often. Moreover, the duration time needs to be read carefully, since the task can now be preempted by threads, I tried to exclude that from the computation, but to keep the probes simple, I didn't exclude interference caused by interrupts. On the same system while isolated, the task runs in about 30-35ms, it is hence highly likely that much larger values are only due to interruptions, rather than the function actually running that long. I will post another email with the scripts used to retrieve the data and more details about the runtime distribution. [1] - https://lore.kernel.org/linux-kernel/20241205083110.180134-2-gmonaco@xxxxxxxxxx/ [2] - https://github.com/arighi/virtme-ng Gabriele Monaco (3): sched: Move task_mm_cid_work to mm delayed work sched: Remove mm_cid_next_scan as obsolete 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 | 157 ++++++++++++++++++ 7 files changed, 203 insertions(+), 79 deletions(-) create mode 100644 tools/testing/selftests/rseq/mm_cid_compaction_test.c base-commit: 231825b2e1ff6ba799c5eaf396d3ab2354e37c6b -- 2.47.1
