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Mon, 20 Mar 2023 22:23:41 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 21 Mar 2023 06:23:39 +0100 To: Dave Chinner Cc: Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Baoquan He , Uladzislau Rezki , Matthew Wilcox , David Hildenbrand , Liu Shixin , Jiri Olsa Subject: Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock Message-ID: References: <6c7f1ac0aeb55faaa46a09108d3999e4595870d9.1679209395.git.lstoakes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D1294180004 X-Stat-Signature: 6twf9gwu94snkinxu6j3zbk5easrcnnb X-HE-Tag: 1679376223-240211 X-HE-Meta: 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 ctk5BFwI 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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: 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! -- Uladzislau Rezki