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Mon, 20 Mar 2023 00:54:35 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 20 Mar 2023 08:54:33 +0100 To: Lorenzo Stoakes Cc: 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: <6c7f1ac0aeb55faaa46a09108d3999e4595870d9.1679209395.git.lstoakes@gmail.com> X-Rspamd-Queue-Id: 9B32480014 X-Stat-Signature: smugxwp7p9zwuj95hpnrnonu41u8cpxp X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1679298877-302377 X-HE-Meta: U2FsdGVkX1/U15Ah4b/jU8ntWiLHTvLHdK6Nb21QUqXF9/c39gxySZC1+N9uU8XXOcVezq/nD9v9M8zNmRl07ZLkU7pzheAF0To53Eu8u5GYptrxwglBpFFUsrJps+Fz55jYQOdnzFUND5MAIoJYrugrgaaiBWWwpnzUuy98bGKxravq5UyzOLrT6dasE0laDPcf0lxtkrBXe7JyXxuz7YfbXl1BTrfj63BQUX+su0tVJBNvMi+v1hiBN9RFRCuHwl9b12IGN9EfZIoRwxsS9MD6OUwxPEKKG9D7tPPlRd4zOPM/EorX2kW62Aql+tq4d5ur/OFiCunmC6peMysxd1jT2V8sHcP63sTKddy8ffzjfa8UBLc7WtWN2Kg0IfhmUaw1gQtckaaVc+sCj2jyKQuwqUcDoSV+enZ4hoVufhNwRoO8g+TQYVhDzUFyl08YSFA4du4irgnjpUtIcz6/gUkqXAt7GaShgXrednMF+mMSsxBneVmX9MssDP74XZ4Hem4PRjIHoa8jiSAd9SgcBn9E4GMsZvEWDom5VJqI3JaWkxQ04f+pONdGk9Jc2n0+0u0HCpUCzuuEjnUOnkwt72kqUTVeOQEPSDMUguhGrONqaj276zTY4NxxFhv1YrTeVgsl+SHu3q7bxYyhsnd8+GvA7HPEpY4ltU2xQr9R6hnxWjfJ+keZHpAK7oL51b16r+DTICrsHsIRhbfxkBIykRkJzERTAX6pw6RxeCRKg3RHKKh1Vh4BPXxV23UsjH0M5BZ310/15ib3y/U09QMIdfA2b/XXvQYEYoovs5Pd9BWa0pZk1EcXUsUUsJIJhMnj/zLZRJDqvW3dfrfV/4FROPsK7KPGh2Q1uA4p/pW+e4mT2GSCO+Xkp0YWlHbCw4klBbG4dBqDzdgJaGY9DvsIxZf0MiQJi3vIxcTmJc99+8WfrN7dRXa8mEIVvdno259hCnUUDHLfMZdyzcWStWb 3hFVYwVg Md2tbT4sp+XyABL26CYMDsz5xLkxVYRRWuCLWufptXqmdxlM9KPLwSMvayU16H+AVHjlASkrwhx42C2p2rWbidw2T+xxtxf26PAg78DLVt/uudPxNzmfL8IKbIHLp5T9dABf6l4VisEaxhbaxaRJMJzsxsYwnHMtnbVn0Q82RtSbt4chu/KV01R0CD58aCKvfnq+6/fliyPDrTh2rK9gvSKOUNFKBj7mQMmfTlcEPrx5BDsJdYDjEgBdvvbrRMApr7Pa/5KSjwJiidcOpvHfO2vO7hK7EfD/d/NskJ3e6popfYiHic1KDaI5y8MKY3YAo9hnj4hRko+wJS+o0G+N8OEJlR9stuC0eiLXEXXkVEsP5Cv68n9yszOucA56jMB/ynXzHcPdMAGosu4sWwO1T7xTyS7lV0EJstxpAE+Y9KA/3+C0GQE6uE8F2VrhShEdft6mfUqS17IxBT/BWr3FXj+oJwWQCT3Gdyej5MzDEh0AaZlE= 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: > 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. > How important to have many simultaneous users of vread()? I do not see a big reason to switch into mutexes due to performance impact and making it less atomic. So, how important for you to have this change? -- Uladzislau Rezki