On Mon, Nov 12, 2018 at 07:30:50AM +0100, Ingo Molnar wrote: > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > On Mon, Nov 12, 2018 at 06:10:33AM +0100, Ingo Molnar wrote: > > > > > > * Waiman Long <longman@xxxxxxxxxx> wrote: > > > > > > > On 11/10/2018 09:10 AM, Peter Zijlstra wrote: > > > > > On Fri, Nov 09, 2018 at 09:04:12AM +0100, Ingo Molnar wrote: > > > > >> BTW., if you are interested in more radical approaches to optimize > > > > >> lockdep, we could also add a static checker via objtool driven call graph > > > > >> analysis, and mark those locks terminal that we can prove are terminal. > > > > >> > > > > >> This would require the unified call graph of the kernel image and of all > > > > >> modules to be examined in a final pass, but that's within the principal > > > > >> scope of objtool. (This 'final pass' could also be done during bootup, at > > > > >> least in initial versions.) > > > > > > > > > > Something like this is needed for objtool LTO support as well. I just > > > > > dread the build time 'regressions' this will introduce :/ > > > > > > > > > > The final link pass is already by far the most expensive part (as > > > > > measured in wall-time) of building a kernel, adding more work there > > > > > would really suck :/ > > > > > > > > I think the idea is to make objtool have the capability to do that. It > > > > doesn't mean we need to turn it on by default in every build. > > > > > > Yeah. > > > > > > Also note that much of the objtool legwork would be on a per file basis > > > which is reasonably parallelized already. On x86 it's also already done > > > for every ORC build i.e. every distro build and the incremental overhead > > > from also extracting locking dependencies should be reasonably small. > > > > > > The final search of the global graph would be serialized but still > > > reasonably fast as these are all 'class' level dependencies which are > > > much less numerous than runtime dependencies. > > > > > > I.e. I think we are talking about tens of thousands of dependencies, not > > > tens of millions. > > > > > > At least in theory. ;-) > > > > Generating a unified call graph sounds very expensive (and very far > > beyond what objtool can do today). > > Well, objtool already goes through the instruction stream and recognizes > function calls - so it can in effect generate a stream of "function x > called by function y" data, correct? Yeah, though it would be quite simple to get the same data with a simple awk script at link time. > > Also, what about function pointers? > > So maybe it's possible to enumerate all potential values for function > pointers with a reasonably simple compiler plugin and work from there? I think this would be somewhere between very difficult and impossible to do properly. I can't even imagine how this would be implemented in a compiler plugin. But I'd love to be proven wrong on that. > One complication would be function pointers encoded as opaque data > types... > > > BTW there's another kernel static analysis tool which attempts to > > create such a call graph already: smatch. > > It's not included in the kernel tree though and I'd expect tight coupling > (or at least lock-step improvements) between tooling and lockdep here. Fair enough. Smatch's call tree isn't perfect anyway, but I don't think perfect is attainable. -- Josh
