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 X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 11BB7C432BE for ; Mon, 2 Aug 2021 21:11:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 8F12160FC0 for ; Mon, 2 Aug 2021 21:11:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 8F12160FC0 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id B60B76B0033; Mon, 2 Aug 2021 17:11:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B11018D0001; Mon, 2 Aug 2021 17:11:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A270D6B005D; Mon, 2 Aug 2021 17:11:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0207.hostedemail.com [216.40.44.207]) by kanga.kvack.org (Postfix) with ESMTP id 861386B0033 for ; Mon, 2 Aug 2021 17:11:16 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 33CFA20430 for ; Mon, 2 Aug 2021 21:11:16 +0000 (UTC) X-FDA: 78431386152.18.4935C77 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf20.hostedemail.com (Postfix) with ESMTP id CB5D1D0027BB for ; Mon, 2 Aug 2021 21:11:15 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id B394260F93; Mon, 2 Aug 2021 21:11:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1627938674; bh=cm7muV9F/ODKslswuJhxzkBNiwyzG0gD+vtM60gPhbE=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=u3I2X+JYXJoZq0hA/qhUG9Ex6O8eXfSWXslSFRa+GFm0QkqtGzdxXkqdaSKXBCMe0 cwuJ5YUeU5con606LctAJk6Pbfw32ShpUriVhaAvUF0/O1moipe2ByJEgcFwK9udsG 9I/XiZkbegLXUcXcqc8IOHj1M679PDRnPvuhkvYw= Date: Mon, 2 Aug 2021 14:11:14 -0700 From: Andrew Morton To: Luigi Rizzo Cc: linux-kernel , David Rientjes , linux-mm@kvack.org Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*() Message-Id: <20210802141114.7fe599b17d87cecbc4d5b70b@linux-foundation.org> In-Reply-To: References: <20210731175341.3458608-1-lrizzo@google.com> <20210801123335.6a7f8e1ee1e52ea64db80323@linux-foundation.org> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=u3I2X+JY; spf=pass (imf20.hostedemail.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: CB5D1D0027BB X-Stat-Signature: kb53xwxmrw3jzba4hyqf35ua6tgeyd8s X-HE-Tag: 1627938675-15695 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 Mon, 2 Aug 2021 02:16:14 +0200 Luigi Rizzo wrote: > > Well, it isn't cost-free. find_vma() is called a lot and a surprising > > number of systems apparently run with CONFIG_DEBUG_VM. Why do you > > think this cost is justified? > > I assume you are concerned with the cost of mmap_assert_locked() ? > > I'd say the justification is the same as for all asserts: > at some point some code change may miss the required lock, and the > asserts are there to catch elusive race conditions, > > There are in fact already instances of mmap_locked_assert() > right before find_vma() in walk_page_range(), and a couple before > calls to __get_user_pages(). > > As for the cost, I'd think that if CONFIG_DEBUG_VM is set, > one does it on purpose to catch errors and is prepared to pay > the cost (in this case the atomic_read(counter) in rwsem_is_locked(), > the counter should be hot). > > FWIW I have instrumented find_vma() on a fast machine using kstats > > https://github.com/luigirizzo/lr-cstats > > (load the module then enable the trace with > echo "trace pcpu:find_vma bits 3" > /sys/kernel/debug/kstats/_control > and monitor the time with > watch "grep CPUS /sys/kernel/debug/kstats/find_vma" > > I didn't run anything especially intensive except some network > benchmarks, but I have collected ~2M samples with the following > distribution of find_vma() time in nanoseconds in 3 configs: > > CONFIGURATION p10 p50 p90 p95 p98 > > no-debug 89 109 214 332 605 > debug 331 369 603 862 1338 > debug+this patch 337 369 603 863 1339 > > As you can see, just compiling a debug kernel, even without this patch, > makes the function 3x more expensive. The effect of this patch is > not measurable (the differences are below measurement error). Cool, thanks, that's convincing.