On Wed, Oct 17, 2018 at 10:23:27PM +0800, Aaron Lu wrote: > > RT has had problems with cpu_relax in the past but more importantly, as > > this delay for parallel compactions and allocations of contig ranges, > > we could be stuck here for very long periods of time with interrupts > > The longest possible time is one CPU accessing pcp->batch number cold > cachelines. Reason: > When zone_wait_cluster_alloc() is called, we already held zone lock so > no more allocations are possible. Waiting in_progress to become zero > means waiting any CPU that increased in_progress to finish processing > their allocated pages. Since they will at most allocate pcp->batch pages > and worse case are all these page structres are cache cold, so the > longest wait time is one CPU accessing pcp->batch number cold cache lines. > > I have no idea if this time is too long though. > But compact_zone calls zone_wait_and_disable_cluster_alloc so how is the disabled time there bound by pcp->batch? > > disabled. It gets even worse if it's from an interrupt context such as > > jumbo frame allocation or a high-order slab allocation that is atomic. > > My understanding is atomic allocation won't trigger compaction, no? > No, they can't. I didn't check properly but be wary of any possibility whereby interrupts can get delayed in zone_wait_cluster_alloc. I didn't go back and check if it can -- partially because I'm more focused on the lazy buddy aspect at the moment. > > It may be necessary to consider instead minimising the number > > of struct page update when merging to PCP and then either increasing the > > size of the PCP or allowing it to exceed pcp->high for short periods of > > time to batch the struct page updates. > > I don't quite follow this part. It doesn't seem possible we can exceed > pcp->high in allocation path, or are you talking about free path? > I'm talking about the free path. > And thanks a lot for the review! My pleasure, hope it helps. -- Mel Gorman SUSE Labs
