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_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 50C40C282CE for ; Tue, 4 Jun 2019 10:20:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2C89E207E0 for ; Tue, 4 Jun 2019 10:20:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727236AbfFDKUN (ORCPT ); Tue, 4 Jun 2019 06:20:13 -0400 Received: from mail-wm1-f65.google.com ([209.85.128.65]:50742 "EHLO mail-wm1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726982AbfFDKUN (ORCPT ); Tue, 4 Jun 2019 06:20:13 -0400 Received: by mail-wm1-f65.google.com with SMTP id f204so10250305wme.0 for ; Tue, 04 Jun 2019 03:20:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=iU0l8vwA6DdSYI8mz2NljjXu7nMKs0q13kn5AwEerD8=; b=k7R2bcZIYlH8kKQKAhqSm73CX9B7zRyAHzP0xtSPS1bKLTrcTG0j6Jn863fNbsEJtd iryHFe0frswuAZc4c5BcFtFmC/im1K1CwEKbJ/ZsQk3tvrmfmFkCPejXKnS0lovBDh54 SGPtY9w0FfuNdf/SE9FmBDDtRtS7c2WpT1LZVo9/Y3PgYvt+bLNeA/iQlBLnoVAja93E RL4jckqnPdBh8/ZDSliH83kx8VihPge08XLYwa+6NSFReeCm7pxPxLIM7zBBCpCPBXT6 FrWewjqjxf02liDwmOS7fvF7lDu2eh83sd6HtpQEj/z5l1vZ0eB/bWEIedHCPL/UsNAH wVfg== X-Gm-Message-State: APjAAAXD//Wf5qoxjlslVk/Srj9GEH3gFovoSRm/UPWRGJOxjL5vZD4x oSUsDt18PU+1YWHQci9VyXoIOA== X-Google-Smtp-Source: APXvYqy+i37B1jRZPK7OZPVrw6iHEB6MIYkUbhzEQj9zo3xW0SHtwco1uSJQMltwP37qXU8rl0Y0xw== X-Received: by 2002:a7b:c5cd:: with SMTP id n13mr18312161wmk.67.1559643610549; Tue, 04 Jun 2019 03:20:10 -0700 (PDT) Received: from t460s.bristot.redhat.com ([5.170.68.106]) by smtp.gmail.com with ESMTPSA id v10sm16083679wml.27.2019.06.04.03.20.08 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Tue, 04 Jun 2019 03:20:09 -0700 (PDT) Subject: Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, williams@redhat.com, daniel@bristot.me, "Steven Rostedt (VMware)" , Ingo Molnar , Thomas Gleixner , "Paul E. McKenney" , Matthias Kaehlcke , "Joel Fernandes (Google)" , Frederic Weisbecker , Yangtao Li , Tommaso Cucinotta References: <20190529083357.GF2623@hirez.programming.kicks-ass.net> <20190529102038.GO2623@hirez.programming.kicks-ass.net> <94669b5a-06dd-e9bf-cfb6-b5d507a90946@redhat.com> <20190529182142.GF2623@hirez.programming.kicks-ass.net> From: Daniel Bristot de Oliveira Message-ID: <9e7850fa-2be5-1dfc-914c-77c9bc9e8c48@redhat.com> Date: Tue, 4 Jun 2019 12:20:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190529182142.GF2623@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/05/2019 20:21, Peter Zijlstra wrote: > On Wed, May 29, 2019 at 03:51:31PM +0200, Daniel Bristot de Oliveira wrote: >> On 29/05/2019 12:20, Peter Zijlstra wrote: > >>> I'm not sure I follow, IRQs disabled fully implies !preemptible. I don't >>> see how the model would be more pessimistic than reality if it were to >>> use this knowledge. >> >> Maybe I did not expressed myself well... and the example was not good either. >> >> "IRQs disabled fully implies !preemptible" is a "to big" step. In modeling (or >> mathematical reasoning?), a good practice is to break the properties into small >> piece, and then build more complex reasoning/implications using these "small >> properties." >> >> Doing "big steps" makes you prone "miss interpretations", creating ambiguity. >> Then, -RT people are prone to be pessimist, non-RT optimistic, and so on... and >> that is what models try to avoid. > > You already construct the big model out of small generators, this is > just one more of those little generators. Yes, we can take that way too... >> For instance, explaining this using words is contradictory:> >>> Any !0 preempt_count(), which very much includes (Hard)IRQ and SoftIRQ >>> counts, means non-preemptible. >> >> One might argue that, the preemption of a thread always takes place with >> preempt_count() != 0, because __schedule() is always called with preemption >> disabled, so the preemption takes place while in non-preemptive. > > Yeah, I know about that one; you've used it in your talks. Also, you've > modeled the schedule preempt disable as a special state. If you want we > can actually make it a special bit in the preempt_count word too, the > patch shouldn't be too hard, although it would make no practical > difference. Good to know! I am trying not to change the code or abstractions. In the user-space version I was using the caller address to figure out if it was a call in the scheduler or not. In the kernel version I am planing to use a is_sched_function() like approach. But if nothing of that works, I will explore this possibility, thanks for the suggestion. >> - WAIT But you (daniel) wants to fake the atomicity between preempt_disable and >> its tracepoint! >> >> Yes, I do, but this is a very straightforward step/assumption: the atomicity is >> about the real-event and the tracepoint that notifies it. It is not about two >> different events. >> >> That is why it is worth letting the modeling rules to clarify the behavior of >> system, without doing non-obvious implication in the code part, so we can have a >> model that fits better in the Linux actions/events to avoid ambiguity. > > You can easily build a little shim betwen the model and the tracehooks > that fix this up if you don't want to stick it in the model proper. > > All the information is there. Heck you can even do that 3/3 thing > internally I think. > Ack, let's see what I can came up. Thanks! -- Daniel