From: Mike Marshall Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing Date: Fri, 25 Nov 2016 15:36:25 -0500 Message-ID: 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> <20161125024918.GX31101@dastard> <20161125041419.GT1555@ZenIV.linux.org.uk> <20161125070642.GZ31101@dastard> <20161125073747.GU1555@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Jan Kara , Andrew Morton , Matthew Wilcox , "linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org" , Dave Chinner , Linux Kernel Mailing List , Steven Rostedt , linux-mm , Ingo Molnar , Al Viro , linux-fsdevel , "linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Christoph Hellwig To: Linus Torvalds Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org > We do have filesystem code that is just disgusting. As an example: > fs/afs/ tends to have these crazy "_enter()/_exit()" macros in every > single function. Hmmm... we have "gossip" statements in Orangefs which can be triggered with a debugfs knob... lots of functions have a gossip statement at the top... is that disgusting? -Mike On Fri, Nov 25, 2016 at 2:51 PM, Linus Torvalds wrote: > On Thu, Nov 24, 2016 at 11:37 PM, Al Viro wrote: >> >> My impression is that nobody (at least kernel-side) wants them to be >> a stable ABI, so long as nobody in userland screams about their code >> being broken, everything is fine. As usual, if nobody notices an ABI >> change, it hasn't happened. The question is what happens when somebody >> does. > > Right. There is basically _no_ "stable API" for the kernel anywhere, > it's just an issue of "you can't break workflow for normal people". > > And if somebody writes his own trace scripts, and some random trace > point goes away (or changes semantics), that's easy: he can just fix > his script. Tracepoints aren't ever going to be stable in that sense. > > But when then somebody writes a trace script that is so useful that > distros pick it up, and people start using it and depending on it, > then _that_ trace point may well have become effectively locked in > stone. > > That's happened once already with the whole powertop thing. It didn't > get all that widely spread, and the fix was largely to improve on > powertop to the point where it wasn't a problem any more, but we've > clearly had one case of this happening. > > But I suspect that something like powertop is fairly unusual. There is > certainly room for similar things in the VFS layer (think "better > vmstat that uses tracepoints"), but I suspect the bulk of tracepoints > are going to be for random debugging (so that developers can say > "please run this script") rather than for an actual user application > kind of situation. > > So I don't think we should be _too_ afraid of tracepoints either. When > being too anti-tracepoint is a bigger practical problem than the > possible problems down the line, the balance is wrong. > > As long as tracepoints make sense from a higher standpoint (ie not > just random implementation detail of the day), and they aren't > everywhere, they are unlikely to cause much problem. > > We do have filesystem code that is just disgusting. As an example: > fs/afs/ tends to have these crazy "_enter()/_exit()" macros in every > single function. If you want that, use the function tracer. That seems > to be just debugging code that has been left around for others to > stumble over. I do *not* believe that we should encourage that kind of > "machine gun spray" use of tracepoints. > > But tracing actual high-level things like IO and faults? I think that > makes perfect sense, as long as the data that is collected is also the > actual event data, and not so much a random implementation issue of > the day. > > Linus > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > More majordomo info at http://vger.kernel.org/majordomo-info.html