On Fri, May 13, 2022 at 12:38:05PM -0700, Andrew Morton wrote: > > The sentence can be dropped because it adds little and is potentially > > confusing. The PCP being safe to access remotely is specific to the > > context of the CPU being hot-removed and there are other special corner > > cases like zone_pcp_disable that modifies a per-cpu structure remotely > > but not in a way that causes corruption. > > OK. I pasted in your para from the other email. Current 0/n blurb: > > Some setups, notably NOHZ_FULL CPUs, may be running realtime or > latency-sensitive applications that cannot tolerate interference due to > per-cpu drain work queued by __drain_all_pages(). Introduce a new > mechanism to remotely drain the per-cpu lists. It is made possible by > remotely locking 'struct per_cpu_pages' new per-cpu spinlocks. This has > two advantages, the time to drain is more predictable and other unrelated > tasks are not interrupted. > > 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 cause a real-time task running > on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining is > 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. > > 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. > Looks good, thanks! -- Mel Gorman SUSE Labs
