On Wed, Apr 20, 2022 at 10:59:00AM +0100, Mel Gorman wrote: > 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. Yeah, the non-deterministic is a problem. I saw the kworker-based draining takes 100+ms(up to 300ms observed) sometimes in alloc_contig_range if CPUs are heavily loaded. I am not sure Nicolas already observed. it's not only problem of per_cpu_pages but it is also lru_pvecs (pagevec) draining. Do we need to introduce similar(allow remote drainning with spin_lock) solution for pagevec? > > 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 > >
