* Waiman Long <longman@xxxxxxxxxx> wrote: > > Could you please measure a locking intense workload instead, such as: > > > > $ perf stat --null --sync --repeat 10 perf bench sched messaging > > > > and profile which locks used there could be marked terminal, and measure > > the before/after performance impact? > > I will run the test. It will probably be done after the LPC next week. Thanks! > >> Below were selected output lines from the lockdep_stats files of the > >> patched and unpatched kernels after bootup and running parallel kernel > >> builds. > >> > >> Item Unpatched kernel Patched kernel % Change > >> ---- ---------------- -------------- -------- > >> direct dependencies 9732 8994 -7.6% > >> dependency chains 18776 17033 -9.3% > >> dependency chain hlocks 76044 68419 -10.0% > >> stack-trace entries 110403 104341 -5.5% > > That's pretty impressive! > > > >> There were some reductions in the size of the lockdep tables. They were > >> not significant, but it is still a good start to rein in the number of > >> entries in those tables to make it harder to overflow them. > > Agreed. > > > > 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.) > > > > Note that beyond marking it 'terminal' such a static analysis pass would > > also allow the detection of obvious locking bugs at the build (or boot) > > stage already - plus it would allow the disabling of lockdep for > > self-contained locks that don't interact with anything else. > > > > I.e. the static analysis pass would 'augment' lockdep and leave only > > those locks active for runtime lockdep tracking whose dependencies it > > cannot prove to be correct yet. > > It is a pretty interesting idea to use objtool to scan for locks. The > list of locks that I marked as terminal in this patch was found by > looking at /proc/lockdep for those that only have backward dependencies, > but no forward dependency. I focused on those with a large number of BDs > and check the code to see if they could marked as terminal. This is a > rather labor intensive process and is subject to error. Yeah. > [...] It would be nice if it can be done by an automated tool. So I am > going to look into that, but it won't be part of this initial patchset, > though. Of course! > I sent this patchset out to see if anyone has any objection to it. It > seems you don't have any objection to that. So I am going to move ahead > to do more testing and performance analysis. The one worry I have is that this interim solution removes the benefit of a proper static analysis method. But if you promise to make a serious effort on the static analysis tooling as well (which should have awesome performance results and automate the manual markup), then I have no fundamental objections to the interim approach either. If static analysis works as well as I expect it to then in principle we might even be able to have lockdep enabled in production kernels: it would only add overhead to locks that are overly complex - which would create incentives to improve those dependencies. Thanks, Ingo
