From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.librecores.org (lists.librecores.org [88.198.125.70]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1E13AC43219 for ; Thu, 26 May 2022 03:41:00 +0000 (UTC) Received: from [172.31.1.100] (localhost.localdomain [127.0.0.1]) by mail.librecores.org (Postfix) with ESMTP id D607F24863; Thu, 26 May 2022 05:40:59 +0200 (CEST) Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by mail.librecores.org (Postfix) with ESMTPS id B01C0247DD for ; Thu, 26 May 2022 05:40:57 +0200 (CEST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 13040615AC; Thu, 26 May 2022 03:40:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5BBA1C385B8; Thu, 26 May 2022 03:40:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1653536455; bh=/FC3fRDbTNCC3O7WmAhxUVAkmggWpUFeLv7OpR1lxLg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=hLLrqk/dSj3ABZgHj+oG01SXfcPdfcPLbCPW5c/vtlKKjGhr826rFN6lLQogyE6R+ V4+rLm9jBH6we4JcRrEMO+7ArvhQ4vUukeFX5qG0umNfJEjOVl7t+dToTe7orX2kZK bSWoDGynO3/vHi04K7UfgRIeRSjHN5GtmmMIX2USGvftGI9pdfeJrjgnAmxrltc5n0 8y833O4MCr4JKKzVPSDGbAvYYjpDQY/dQCqYoM1yOIlCVTJtDFNiHcFx0Vs05+lAF0 ALTHrfJeOAfPcImvCexeRL9L6Mpy6p/bjPRzg8B1971s8Zq8YHz1AyoKFvxIGL0AWO PHAjKqVSOf5bw== Message-ID: <8f6add25-2e8f-4533-fa42-e43db0e32f2d@kernel.org> Date: Wed, 25 May 2022 20:40:51 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.8.1 Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Content-Language: en-US To: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20220524234531.1949-1-peterx@redhat.com> From: Vineet Gupta In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: openrisc@lists.librecores.org X-Mailman-Version: 2.1.26 Precedence: list List-Id: Discussion around the OpenRISC processor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: x86@kernel.org, Catalin Marinas , David Hildenbrand , Peter Zijlstra , Benjamin Herrenschmidt , Dave Hansen , "James E . J . Bottomley" , Max Filippov , Rich Felker , Paul Mackerras , "H . Peter Anvin" , sparclinux@vger.kernel.org, linux-ia64@vger.kernel.org, Alexander Gordeev , Will Deacon , linux-riscv@lists.infradead.org, Anton Ivanov , Jonas Bonn , linux-s390@vger.kernel.org, linux-snps-arc@lists.infradead.org, Yoshinori Sato , linux-xtensa@linux-xtensa.org, linux-hexagon@vger.kernel.org, Helge Deller , Alistair Popple , Hugh Dickins , Russell King , linux-csky@vger.kernel.org, linux-sh@vger.kernel.org, Ingo Molnar , linux-arm-kernel@lists.infradead.org, Vineet Gupta , Matt Turner , Christian Borntraeger , Andrea Arcangeli , Albert Ou , Vasily Gorbik , Brian Cain , Heiko Carstens , Johannes Weiner , linux-um@lists.infradead.org, Nicholas Piggin , Richard Weinberger , linux-m68k@lists.linux-m68k.org, openrisc@lists.librecores.org, Ivan Kokshaysky , Al Viro , Andy Lutomirski , Paul Walmsley , Thomas Gleixner , linux-alpha@vger.kernel.org, Andrew Morton , Vlastimil Babka , Richard Henderson , Chris Zankel , Michal Simek , Thomas Bogendoerfer , linux-parisc@vger.kernel.org, linux-mips@vger.kernel.org, Dinh Nguyen , Palmer Dabbelt , Sven Schnelle , Guo Ren , Michael Ellerman , Borislav Petkov , Johannes Berg , linuxppc-dev@lists.ozlabs.org, "David S . Miller" Errors-To: openrisc-bounces@lists.librecores.org Sender: "OpenRISC" On 5/24/22 16:45, Peter Xu wrote: > I observed that for each of the shared file-backed page faults, we're very > likely to retry one more time for the 1st write fault upon no page. It's > because we'll need to release the mmap lock for dirty rate limit purpose > with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()). > > Then after that throttling we return VM_FAULT_RETRY. > > We did that probably because VM_FAULT_RETRY is the only way we can return > to the fault handler at that time telling it we've released the mmap lock. > > However that's not ideal because it's very likely the fault does not need > to be retried at all since the pgtable was well installed before the > throttling, so the next continuous fault (including taking mmap read lock, > walk the pgtable, etc.) could be in most cases unnecessary. > > It's not only slowing down page faults for shared file-backed, but also add > more mmap lock contention which is in most cases not needed at all. > > To observe this, one could try to write to some shmem page and look at > "pgfault" value in /proc/vmstat, then we should expect 2 counts for each > shmem write simply because we retried, and vm event "pgfault" will capture > that. > > To make it more efficient, add a new VM_FAULT_COMPLETED return code just to > show that we've completed the whole fault and released the lock. It's also > a hint that we should very possibly not need another fault immediately on > this page because we've just completed it. > > 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%) > > I believe it could help more than that. > > We need some special care on GUP and the s390 pgfault handler (for gmap > code before returning from pgfault), the rest changes in the page fault > handlers should be relatively straightforward. > > Another thing to mention is that mm_account_fault() does take this new > fault as a generic fault to be accounted, unlike VM_FAULT_RETRY. > > I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do > not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping > them as-is. > > Signed-off-by: Peter Xu > --- > > v3: > - Rebase to akpm/mm-unstable > - Copy arch maintainers > --- > arch/arc/mm/fault.c | 4 ++++ Acked-by: Vineet Gupta Thx, -Vineet