From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id EBD246B0069 for ; Thu, 24 Nov 2016 21:49:23 -0500 (EST) Received: by mail-pf0-f198.google.com with SMTP id j128so89092282pfg.4 for ; Thu, 24 Nov 2016 18:49:23 -0800 (PST) Received: from ipmail05.adl6.internode.on.net (ipmail05.adl6.internode.on.net. [150.101.137.143]) by mx.google.com with ESMTP id q187si30060945pfb.256.2016.11.24.18.49.21 for ; Thu, 24 Nov 2016 18:49:22 -0800 (PST) Date: Fri, 25 Nov 2016 13:49:18 +1100 From: Dave Chinner Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing Message-ID: <20161125024918.GX31101@dastard> References: <1479926662-21718-1-git-send-email-ross.zwisler@linux.intel.com> <1479926662-21718-4-git-send-email-ross.zwisler@linux.intel.com> <20161124173220.GR1555@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161124173220.GR1555@ZenIV.linux.org.uk> Sender: owner-linux-mm@kvack.org List-ID: To: Al Viro Cc: Ross Zwisler , linux-kernel@vger.kernel.org, Andrew Morton , Christoph Hellwig , Dan Williams , Ingo Molnar , Jan Kara , Matthew Wilcox , Steven Rostedt , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org On Thu, Nov 24, 2016 at 05:32:20PM +0000, Al Viro wrote: > On Wed, Nov 23, 2016 at 11:44:19AM -0700, Ross Zwisler wrote: > > Tracepoints are the standard way to capture debugging and tracing > > information in many parts of the kernel, including the XFS and ext4 > > filesystems. Create a tracepoint header for FS DAX and add the first DAX > > tracepoints to the PMD fault handler. This allows the tracing for DAX to > > be done in the same way as the filesystem tracing so that developers can > > look at them together and get a coherent idea of what the system is doing. > > It also has one hell of potential for becoming a massive nuisance. > Keep in mind that if any userland code becomes dependent on those - that's it, > they have become parts of stable userland ABI and are to be maintained > indefinitely. Don't expect "tracepoints are special case" to prevent that. I call bullshit just like I always do when someone spouts this "tracepoints are stable ABI" garbage. If we want to provide stable tracepoints, then we need to /create a stable tracepoint API/ and convert all the tracepoints that /need to be stable/ to use it. Then developers only need to be careful about modifying code around the /explicitly stable/ tracepoints and we avoid retrospectively locking the kernel implementation into a KABI so tight we can't do anything anymore.... Quite frankly, anyone that wants to stop us from adding/removing/changing tracepoints or the code that they are reporting information about "because ABI" can go take a long walk off a short cliff. Diagnostic tracepoints are not part of the stable ABI. End of story. > So treat anything you add in that manner as potential stable ABI > you might have to keep around forever. It's *not* a glorified debugging > printk. trace_printk() is the glorified debugging printk for tracing, not trace events. Cheers, Dave. -- Dave Chinner david@fromorbit.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org