On Tue, Mar 21, 2023 at 09:05:26PM +1100, Dave Chinner wrote: > On Tue, Mar 21, 2023 at 07:45:56AM +0000, Lorenzo Stoakes wrote: > > 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/ > > Yeah, I'd already read that. > > What I want to do, though, is to determine whether the problem > shared access contention or exclusive access contention. If it's > exclusive access contention, then an rwsem will do nothing to > alleviate the problem, and that's kinda critical to know before any > fix for the contention problems are worked out... > > > > 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. > > If you want data about contention problems with vmalloc > > > > Requirements: > > > * It should be a per-cpu approach; > > Hmmmm. My 2c worth on this: That is not a requirement. > > That's a -solution-. > > The requirement is that independent concurrent vmalloc/vfree > operations do not severely contend with each other. > > Yes, the solution will probably involve sharding the resource space > across mulitple independent structures (as we do in filesystems with > block groups, allocations groups, etc) but that does not necessarily > need the structures to be per-cpu. > > e.g per-node vmalloc arenas might be sufficient and allow more > expensive but more efficient indexing structures to be used because > we don't have to care about the explosion of memory that > fine-grained per-cpu indexing generally entails. This may also fit > in to the existing per-node structure of the memory reclaim > infrastructure to manage things like compaction, balancing, etc of > vmalloc space assigned to the given node. > > Hence I think saying "per-cpu is a requirement" kinda prevents > exploration of other novel solutions that may have advantages other > than "just solves the concurrency problem"... > > > > * 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 > > I'd call these desired traits and/or potential optimisations, not > hard requirements. > > > > Goals: > > > * Implement a per-cpu way of allocation to eliminate a contention. > > The goal should be to "allow contention-free vmalloc operations", not > that we implement a specific solution. > I think we are on the same page. I do not see that we go apart in anything. Probably i was a bit more specific in requirements but this is how i see personally on it based on different kind of experiments with it. Thank you for your 2c! -- Uladzislau Rezki
