From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f197.google.com (mail-qk1-f197.google.com [209.85.222.197]) by kanga.kvack.org (Postfix) with ESMTP id 7DEBB6B0006 for ; Mon, 12 Nov 2018 17:23:01 -0500 (EST) Received: by mail-qk1-f197.google.com with SMTP id k203so27024360qke.2 for ; Mon, 12 Nov 2018 14:23:01 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id k18si3196206qtj.92.2018.11.12.14.23.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 12 Nov 2018 14:23:00 -0800 (PST) Date: Mon, 12 Nov 2018 16:22:50 -0600 From: Josh Poimboeuf Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks Message-ID: <20181112222250.h37hkrj6warqewkd@treble> References: <1541709268-3766-1-git-send-email-longman@redhat.com> <20181109080412.GC86700@gmail.com> <20181110141045.GD3339@worktop.programming.kicks-ass.net> <20181112051033.GA123204@gmail.com> <20181112055324.f7div2ahx5emkbbe@treble> <20181112063050.GB61749@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20181112063050.GB61749@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Ingo Molnar Cc: Waiman Long , 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 On Mon, Nov 12, 2018 at 07:30:50AM +0100, Ingo Molnar wrote: > > * Josh Poimboeuf wrote: > > > On Mon, Nov 12, 2018 at 06:10:33AM +0100, Ingo Molnar wrote: > > > > > > * Waiman Long wrote: > > > > > > > On 11/10/2018 09:10 AM, Peter Zijlstra wrote: > > > > > On Fri, Nov 09, 2018 at 09:04:12AM +0100, Ingo Molnar wrote: > > > > >> 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.) > > > > > > > > > > Something like this is needed for objtool LTO support as well. I just > > > > > dread the build time 'regressions' this will introduce :/ > > > > > > > > > > The final link pass is already by far the most expensive part (as > > > > > measured in wall-time) of building a kernel, adding more work there > > > > > would really suck :/ > > > > > > > > I think the idea is to make objtool have the capability to do that. It > > > > doesn't mean we need to turn it on by default in every build. > > > > > > Yeah. > > > > > > Also note that much of the objtool legwork would be on a per file basis > > > which is reasonably parallelized already. On x86 it's also already done > > > for every ORC build i.e. every distro build and the incremental overhead > > > from also extracting locking dependencies should be reasonably small. > > > > > > The final search of the global graph would be serialized but still > > > reasonably fast as these are all 'class' level dependencies which are > > > much less numerous than runtime dependencies. > > > > > > I.e. I think we are talking about tens of thousands of dependencies, not > > > tens of millions. > > > > > > At least in theory. ;-) > > > > Generating a unified call graph sounds very expensive (and very far > > beyond what objtool can do today). > > Well, objtool already goes through the instruction stream and recognizes > function calls - so it can in effect generate a stream of "function x > called by function y" data, correct? Yeah, though it would be quite simple to get the same data with a simple awk script at link time. > > Also, what about function pointers? > > So maybe it's possible to enumerate all potential values for function > pointers with a reasonably simple compiler plugin and work from there? I think this would be somewhere between very difficult and impossible to do properly. I can't even imagine how this would be implemented in a compiler plugin. But I'd love to be proven wrong on that. > One complication would be function pointers encoded as opaque data > types... > > > BTW there's another kernel static analysis tool which attempts to > > create such a call graph already: smatch. > > It's not included in the kernel tree though and I'd expect tight coupling > (or at least lock-step improvements) between tooling and lockdep here. Fair enough. Smatch's call tree isn't perfect anyway, but I don't think perfect is attainable. -- Josh