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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 736C2C43441 for ; Fri, 9 Nov 2018 15:48:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3D86D20825 for ; Fri, 9 Nov 2018 15:48:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3D86D20825 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728054AbeKJB3t convert rfc822-to-8bit (ORCPT ); Fri, 9 Nov 2018 20:29:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40112 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727784AbeKJB3t (ORCPT ); Fri, 9 Nov 2018 20:29:49 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BCD72C0495B5; Fri, 9 Nov 2018 15:48:41 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-55.bos.redhat.com [10.18.17.55]) by smtp.corp.redhat.com (Postfix) with ESMTP id C6E682BC70; Fri, 9 Nov 2018 15:48:39 +0000 (UTC) Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks To: Ingo Molnar , Josh Poimboeuf Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, linux-mm@kvack.org, Petr Mladek , Sergey Senozhatsky , Andrey Ryabinin , Tejun Heo , Andrew Morton , Linus Torvalds References: <1541709268-3766-1-git-send-email-longman@redhat.com> <20181109080412.GC86700@gmail.com> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181109080412.GC86700@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Fri, 09 Nov 2018 15:48:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/09/2018 03:04 AM, Ingo Molnar wrote: > * Waiman Long wrote: > >> The purpose of this patchset is to add a new class of locks called >> terminal locks and converts some of the low level raw or regular >> spinlocks to terminal locks. A terminal lock does not have forward >> dependency and it won't allow a lock or unlock operation on another >> lock. Two level nesting of terminal locks is allowed, though. >> >> Only spinlocks that are acquired with the _irq/_irqsave variants or >> acquired in an IRQ disabled context should be classified as terminal >> locks. >> >> Because of the restrictions on terminal locks, we can do simple checks on >> them without using the lockdep lock validation machinery. The advantages >> of making these changes are as follows: >> >> 1) The lockdep check will be faster for terminal locks without using >> the lock validation code. >> 2) It saves table entries used by the validation code and hence make >> it harder to overflow those tables. >> >> In fact, it is possible to overflow some of the tables by running >> a variety of different workloads on a debug kernel. I have seen bug >> reports about exhausting MAX_LOCKDEP_KEYS, MAX_LOCKDEP_ENTRIES and >> MAX_STACK_TRACE_ENTRIES. This patch will help to reduce the chance >> of overflowing some of the tables. >> >> Performance wise, there was no statistically significant difference in >> performanace when doing a parallel kernel build on a debug kernel. > Could you please measure a locking intense workload instead, such as: > > $ perf stat --null --sync --repeat 10 perf bench sched messaging > > and profile which locks used there could be marked terminal, and measure > the before/after performance impact? I will run the test. It will probably be done after the LPC next week. >> Below were selected output lines from the lockdep_stats files of the >> patched and unpatched kernels after bootup and running parallel kernel >> builds. >> >> Item Unpatched kernel Patched kernel % Change >> ---- ---------------- -------------- -------- >> direct dependencies 9732 8994 -7.6% >> dependency chains 18776 17033 -9.3% >> dependency chain hlocks 76044 68419 -10.0% >> stack-trace entries 110403 104341 -5.5% > That's pretty impressive! > >> There were some reductions in the size of the lockdep tables. They were >> not significant, but it is still a good start to rein in the number of >> entries in those tables to make it harder to overflow them. > Agreed. > > BTW., if you are interested in more radical approaches to optimize > lockdep, we could also add a static checker via objtool driven call graph > analysis, and mark those locks terminal that we can prove are terminal. > > This would require the unified call graph of the kernel image and of all > modules to be examined in a final pass, but that's within the principal > scope of objtool. (This 'final pass' could also be done during bootup, at > least in initial versions.) > > Note that beyond marking it 'terminal' such a static analysis pass would > also allow the detection of obvious locking bugs at the build (or boot) > stage already - plus it would allow the disabling of lockdep for > self-contained locks that don't interact with anything else. > > I.e. the static analysis pass would 'augment' lockdep and leave only > those locks active for runtime lockdep tracking whose dependencies it > cannot prove to be correct yet. It is a pretty interesting idea to use objtool to scan for locks. The list of locks that I marked as terminal in this patch was found by looking at /proc/lockdep for those that only have backward dependencies, but no forward dependency. I focused on those with a large number of BDs and check the code to see if they could marked as terminal. This is a rather labor intensive process and is subject to error. It would be nice if it can be done by an automated tool. So I am going to look into that, but it won't be part of this initial patchset, though. I sent this patchset out to see if anyone has any objection to it. It seems you don't have any objection to that. So I am going to move ahead to do more testing and performance analysis. Thanks, Longman