On Mon, Apr 25, 2022 at 7:49 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Wed, Apr 20, 2022 at 2:59 AM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> 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. > > > > 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. > > This quite possibly solves the issue I was trying to fix in > https://lore.kernel.org/all/20220225012819.1807147-1-surenb@xxxxxxxxxx. > I will give it a try and see how it looks. My test shows sizable improvement for the worst case drain_all_pages duration. Before the change I caught cases when a drain_local_pages_wq in the workqueue was delayed by 100+ms (not even counting drain_local_pages_wq execution time itself). With this patchset the worst time I was able to record for drain_all_pages duration was 17ms. > Thanks! > > > > > 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 > > > >
