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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DB32FC433F5 for ; Wed, 25 May 2022 11:11:26 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6854A8D0005; Wed, 25 May 2022 07:11:26 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 658408D0001; Wed, 25 May 2022 07:11:26 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 521AF8D0005; Wed, 25 May 2022 07:11:26 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 40F058D0001 for ; Wed, 25 May 2022 07:11:26 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay13.hostedemail.com (Postfix) with ESMTP id 10BD7611A8 for ; Wed, 25 May 2022 11:11:26 +0000 (UTC) X-FDA: 79503999372.02.AB41EE6 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) by imf13.hostedemail.com (Postfix) with ESMTP id C7C7720021 for ; Wed, 25 May 2022 11:10:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=TTSfncRF4bwVxkeR+S4pHa2NuE5vU7NkHqU52imXoeo=; b=AZ7NLjulpP1WzqY4jAw9CJ6mzn eNxn6mITLdAkzreesFIusYyJANkaOvbZVslHzJxDDqo/jW9prJ6mSOYajnNAsWgSTZTPCGe4Y+H+t Q7b5K2i3OxODvn+feDcYZuuvagMcqlznlm2xJ6e8wPUzB4c/WkijGr50Z8D32ZlBmwWuMwGQbue1E mrsCyjPGVmj7xf/0/rnV0zIseeyVAHCeyTCYTi0ZFmZwkk1CWC0IMPFglArv51u64lwHpRP8/QuNm TypGtB34O3uGaPoduTA8k7e4jOZswva+y2onGqiOIb4Z5GAtPEnJDGpyLSzDy3ZlKvUO8MvNVaZm2 4u9mbK6A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1ntouF-001b5f-PS; Wed, 25 May 2022 11:10:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 5B9C13002AF; Wed, 25 May 2022 13:10:17 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 374CD207688E7; Wed, 25 May 2022 13:10:17 +0200 (CEST) Date: Wed, 25 May 2022 13:10:17 +0200 From: Peter Zijlstra 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 . 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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 , 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" Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Message-ID: References: <20220524234531.1949-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C7C7720021 X-Stat-Signature: b9quj1dj3c6k6xpr8zshcm9wruz5cz63 X-Rspam-User: Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=infradead.org header.s=desiato.20200630 header.b=AZ7NLjul; spf=none (imf13.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.92.199) smtp.mailfrom=peterz@infradead.org; dmarc=none X-HE-Tag: 1653477057-48487 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, May 24, 2022 at 07:45:31PM -0400, 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 Acked-by: Peter Zijlstra (Intel)