Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks

[Ingo Molnar]

* 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