This series has the same intent as Nicolas' series "mm/page_alloc: Remote per-cpu lists drain support" -- avoid interference of a high priority task due to a workqueue item draining per-cpu page lists. While many workloads can tolerate a brief interruption, it may be cause a real-time task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining in non-deterministic. Currently an IRQ-safe local_lock protects the page allocator per-cpu lists. The local_lock on its own prevents migration and the IRQ disabling protects from corruption due to an interrupt arriving while a page allocation is in progress. The locking is inherently unsafe for remote access unless the CPU is hot-removed. This series adjusts the locking. A spin-lock is added to struct per_cpu_pages to protect the list contents while local_lock_irq continues to prevent migration and IRQ reentry. This allows a remote CPU to safely drain a remote per-cpu list. This series is a partial series. Follow-on work would allow the local_irq_save to be converted to a local_irq to avoid IRQs being disabled/enabled in most cases. However, there are enough corner cases that it deserves a series on its own separated by one kernel release and the priority right now is to avoid interference of high priority tasks. Patch 1 is a cosmetic patch to clarify when page->lru is storing buddy pages and when it is storing per-cpu pages. Patch 2 shrinks per_cpu_pages to make room for a spin lock. Strictly speaking this is not necessary but it avoids per_cpu_pages consuming another cache line. Patch 3 is a preparation patch to avoid code duplication. Patch 4 is a simple micro-optimisation that improves code flow necessary for a later patch to avoid code duplication. Patch 5 uses a spin_lock to protect the per_cpu_pages contents while still relying on local_lock to prevent migration, stabilise the pcp lookup and prevent IRQ reentrancy. Patch 6 remote drains per-cpu pages directly instead of using a workqueue. include/linux/mm_types.h | 5 + include/linux/mmzone.h | 12 +- mm/page_alloc.c | 333 ++++++++++++++++++++++++--------------- 3 files changed, 222 insertions(+), 128 deletions(-) -- 2.34.1
