On Tue, Mar 21, 2023 at 12:09:12PM +1100, Dave Chinner wrote: > On Sun, Mar 19, 2023 at 07:09:31AM +0000, Lorenzo Stoakes wrote: > > vmalloc() is, by design, not permitted to be used in atomic context and > > already contains components which may sleep, so avoiding spin locks is not > > a problem from the perspective of atomic context. > > > > The global vmap_area_lock is held when the red/black tree rooted in > > vmap_are_root is accessed and thus is rather long-held and under > > potentially high contention. It is likely to be under contention for reads > > rather than write, so replace it with a rwsem. > > > > Each individual vmap_block->lock is likely to be held for less time but > > under low contention, so a mutex is not an outrageous choice here. > > > > A subset of test_vmalloc.sh performance results:- > > > > fix_size_alloc_test 0.40% > > full_fit_alloc_test 2.08% > > long_busy_list_alloc_test 0.34% > > random_size_alloc_test -0.25% > > random_size_align_alloc_test 0.06% > > ... > > all tests cycles 0.2% > > > > This represents a tiny reduction in performance that sits barely above > > noise. > > I'm travelling right now, but give me a few days and I'll test this > against the XFS workloads that hammer the global vmalloc spin lock > really, really badly. XFS can use vm_map_ram and vmalloc really > heavily for metadata buffers and hit the global spin lock from every > CPU in the system at the same time (i.e. highly concurrent > workloads). vmalloc is also heavily used in the hottest path > throught the journal where we process and calculate delta changes to > several million items every second, again spread across every CPU in > the system at the same time. > > We really need the global spinlock to go away completely, but in the > mean time a shared read lock should help a little bit.... > I am working on it. I submitted a proposal how to eliminate it: <snip> Hello, LSF. Title: Introduce a per-cpu-vmap-cache to eliminate a vmap lock contention Description: Currently the vmap code is not scaled to number of CPU cores in a system because a global vmap space is protected by a single spinlock. Such approach has a clear bottleneck if many CPUs simultaneously access to one resource. In this talk i would like to describe a drawback, show some data related to contentions and places where those occur in a code. Apart of that i would like to share ideas how to eliminate it providing a few approaches and compare them. Requirements: * It should be a per-cpu approach; * Search of freed ptrs should not interfere with other freeing(as much as we can); * - offload allocated areas(buzy ones) per-cpu; * Cache ready sized objects or merge them into one big per-cpu-space(split on demand); * Lazily-freed areas either drained per-cpu individually or by one CPU for all; * Prefetch a fixed size in front and allocate per-cpu Goals: * Implement a per-cpu way of allocation to eliminate a contention. Thanks! <snip> -- Uladzislau Rezki
