On Thu, May 26, 2022 at 01:19:38PM -0400, Qian Cai wrote: > On Thu, May 12, 2022 at 09:50:37AM +0100, Mel Gorman wrote: > > Changelog since v2 > > o More conversions from page->lru to page->[pcp_list|buddy_list] > > o Additional test results in changelogs > > > > Changelog since v1 > > o Fix unsafe RT locking scheme > > o Use spin_trylock on UP PREEMPT_RT > > > > 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 spinlock 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 should allow the > > local_irq_save to be converted to a local_irq to avoid IRQs being > > disabled/enabled in most cases. Consequently, there are some TODO comments > > highlighting the places that would change if local_irq was used. 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. > > Mel, we saw spontanous "mm_percpu_wq" crash on today's linux-next tree > while running CPU offlining/onlining, and wondering if you have any > thoughts? > Do you think it's related to the series and if so why? From the warning, it's not obvious to me why it would be given that it's a warning about a task not being inactive when it is expected to be. -- Mel Gorman SUSE Labs
