On Tue, Mar 21, 2023 at 06:23:39AM +0100, Uladzislau Rezki wrote: > 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.... > > Hugely appreciated Dave, however I must disappoint on the rwsem as I have now reworked my patch set to use the original locks in order to satisfy Willy's desire to make vmalloc atomic in future, and Uladzislau's desire to not have a ~6% performance hit - https://lore.kernel.org/all/cover.1679354384.git.lstoakes@xxxxxxxxx/ > 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 > That's really awesome! I will come to that talk at LSF/MM :) being able to sustain the lock in atomic context seems to be an aspect that is important going forward also.
