> 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 <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, Nov 24, 2016 at 11:37 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
