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=ham 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 06D3CC4338F for ; Mon, 2 Aug 2021 21:11:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E22D260F70 for ; Mon, 2 Aug 2021 21:11:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232521AbhHBVLZ (ORCPT ); Mon, 2 Aug 2021 17:11:25 -0400 Received: from mail.kernel.org ([198.145.29.99]:38900 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232532AbhHBVLY (ORCPT ); Mon, 2 Aug 2021 17:11:24 -0400 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.