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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 2CE6DC433DF for ; Tue, 30 Jun 2020 08:57:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F02B320759 for ; Tue, 30 Jun 2020 08:57:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ShU9An0u" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731781AbgF3I5i (ORCPT ); Tue, 30 Jun 2020 04:57:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58878 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731639AbgF3I5i (ORCPT ); Tue, 30 Jun 2020 04:57:38 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B26BFC061755; Tue, 30 Jun 2020 01:57:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=kE9wxlddwelBGVB/Y6TI6m2LRXPO5/frNH3uoUED72o=; b=ShU9An0u1w5AzvipJCcUbeNI9b MdhYFhcrV+E2X+p4a4ql+tl2fHf7oObOwZZCNPKx7lte4sB4MCcz0lpcjos2oopZP9i7GKJcfiyeb QqoCkYF/OQOH4YvwiZB8s9Wh4ObnqHh6fn0bCA7YWTlm/gp/utUCOuMGkPhKhhD3iSRtJ8bS2qZhH /hhMIr73JitY6j+4inkz/lTwsn+ScghAXKKHGBAYPDtknH1stNAHWi8DBSKPl8T291T73hD0VXVDM abP3QH5s5Fw44qXOOZmxmP6RtRRtNdjafgjohbQt4B6sNZ/PsNpUjMHYoBIk6IuKKfswPF6oeJ0y4 vd+OfvVw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jqC59-0001ow-Ed; Tue, 30 Jun 2020 08:57:35 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id B0DEC305B36; Tue, 30 Jun 2020 10:57:32 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9A69421405126; Tue, 30 Jun 2020 10:57:32 +0200 (CEST) Date: Tue, 30 Jun 2020 10:57:32 +0200 From: Peter Zijlstra To: Dave Chinner Cc: linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, Ingo Molnar Subject: Re: [Bug, sched, 5.8-rc2]: PREEMPT kernels crashing in check_preempt_wakeup() running fsx on XFS Message-ID: <20200630085732.GT4817@hirez.programming.kicks-ass.net> References: <20200626004722.GF2005@dread.disaster.area> <20200626073345.GI4800@hirez.programming.kicks-ass.net> <20200626223254.GH2005@dread.disaster.area> <20200627183042.GK4817@hirez.programming.kicks-ass.net> <20200629235533.GL2005@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200629235533.GL2005@dread.disaster.area> Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Jun 30, 2020 at 09:55:33AM +1000, Dave Chinner wrote: > Sure, but that misses the point I was making. > > I regularly have to look deep into other subsystems to work out what > problem the filesystem is tripping over. I'm regularly > looking into parts of the IO stack, memory management, page > allocators, locking and atomics, workqueues, the scheduler, etc > because XFS makes extensive (and complex) use of the infrastructure > they provide. That means to debug filesystem issues, I have to be > able to understand what that infrastructure is trying to do and make > judgements as to whether that code behaving correctly or not. > > And so when I find a reproducer for a bug that takes 20s to > reproduce and it points me at code that I honestily have no hope of 20s would've been nice to have a week and a half ago, the reproduce I debugged this with took days to trigger.. a well, such is life. > understanding well enough to determine if it is working correctly or > not, then we have a problem. A lot of my time is spent doing root > cause analysis proving that such issues are -not- filesystem > problems (they just had "xfs" in the stack trace), hence being able > to read and understand the code in related core subsystems is > extremely important to performing my day job. > > If more kernel code falls off the memory barrier cliff like this, > then the ability of people like me to find the root cause of complex > issues is going to be massively reduced. Writing code so smart that > almost no-one else can understand has always been a bad thing, and > memory barriers only make this problem worse... :( How about you try and give me a hint about where you gave up and I'll try and write better comments?