From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Date: Fri, 27 May 2022 10:46:31 +0000 Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Message-Id: List-Id: References: <20220524234531.1949-1-peterx@redhat.com> In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Richard Henderson , David Hildenbrand , Matt Turner , Albert Ou , Michal Simek , Russell King , Ivan Kokshaysky , linux-riscv@lists.infradead.org, Alexander Gordeev , Dave Hansen , Jonas Bonn , Will Deacon , "James E . J . Bottomley" , "H . Peter Anvin" , Andrea Arcangeli , openrisc@lists.librecores.org, linux-s390@vger.kernel.org, Ingo Molnar , linux-m68k@lists.linux-m68k.org, Palmer Dabbelt , Heiko Carstens , Chris Zankel , Peter Zijlstra , Alistair Popple , linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, Vlastimil Babka , Thomas Gleixner , sparclinux@vger.kernel.org, Christian Borntraeger , Stafford Horne , Michael Ellerman , x86@kernel.org, Thomas Bogendoerfer , Paul Mackerras , linux-arm-kernel@lists.infradead.org, Sven Schnelle , Benjamin Herrenschmidt , linux-xtensa@linux-xtensa.org, Nicholas Piggin , linux-sh@vger.kernel.org, Vasily Gorbik , Borislav Petkov , linux-mips@vger.kernel.org, Max Filippov , Helge Deller , Vineet Gupta , Al Viro , Paul Walmsley , Johannes Weiner , Anton Ivanov , Catalin Marinas , linux-um@lists.infradead.org, linux-alpha@vger.kernel.org, Johannes Berg , linux-ia64@vger.kernel.org, Geert Uytterhoeven , Dinh Nguyen , Guo Ren , linux-snps-arc@lists.infradead.org, Hugh Dickins , Rich Felker , Andy Lutomirski , Richard Weinberger , linuxppc-dev@lists.ozlabs.org, Brian Cain , Yoshinori Sato , Andrew Morton , Stefan Kristiansson , linux-parisc@vger.kernel.org, "David S . Miller" * Peter Xu wrote: > This patch provides a ~12% perf boost on my aarch64 test VM with a simple > program sequentially dirtying 400MB shmem file being mmap()ed and these are > the time it needs: > > Before: 650.980 ms (+-1.94%) > After: 569.396 ms (+-1.38%) Nice! > arch/x86/mm/fault.c | 4 ++++ Reviewed-by: Ingo Molnar Minor comment typo: > + /* > + * We should do the same as VM_FAULT_RETRY, but let's not > + * return -EBUSY since that's not reflecting the reality on > + * what has happened - we've just fully completed a page > + * fault, with the mmap lock released. Use -EAGAIN to show > + * that we want to take the mmap lock _again_. > + */ s/reflecting the reality on what has happened /reflecting the reality of what has happened > ret = handle_mm_fault(vma, address, fault_flags, NULL); > + > + if (ret & VM_FAULT_COMPLETED) { > + /* > + * NOTE: it's a pity that we need to retake the lock here > + * to pair with the unlock() in the callers. Ideally we > + * could tell the callers so they do not need to unlock. > + */ > + mmap_read_lock(mm); > + *unlocked = true; > + return 0; Indeed that's a pity - I guess more performance could be gained here, especially in highly parallel threaded workloads? Thanks, Ingo