Dramatic lockdep slowdown in 4.14

[Johan Hovold]

On Fri, Oct 13, 2017 at 11:07:44AM +0200, Peter Zijlstra wrote:
> On Fri, Oct 13, 2017 at 11:03:33AM +0200, Johan Hovold wrote:
> > Hi,
> > 
> > I had noticed that the BeagleBone Black boot time appeared to have
> > increased significantly with 4.14 and yesterday I finally had time to
> > investigate it.
> > 
> > Boot time (from "Linux version" to login prompt) had in fact doubled
> > since 4.13 where it took 17 seconds (with my current config) compared to
> > the 35 seconds I now see with 4.14-rc4.
> > 
> > I quick bisect pointed to lockdep and specifically the following commit:
> > 
> > 	28a903f63ec0 ("locking/lockdep: Handle non(or multi)-acquisition
> > 	               of a crosslock")
> > 
> > which I've verified is the commit which doubled the boot time (compared
> > to 28a903f63ec0^) (added by lockdep crossrelease series [1]).
> > 
> > I also verified that simply disabling CONFIG_PROVE_LOCKING on 4.14-rc4
> > brought boot time down to about 14 seconds.
> > 
> > Now since it's lockdep I guess this can't really be considered a
> > regression if these changes did improve lockdep correctness, but still,
> > this dramatic slow down essentially forces me to disable PROVE_LOCKING
> > by default on this system.
> > 
> > Is this lockdep slowdown expected and desirable?
> 
> Expected yes, desirable not so much. Its the save_stack_trace() in
> add_xhlock() (IIRC).
> 
> I've not yet had time to figure out what to do about that.

Thanks for confirming. Do you think it makes sense to track this as a
4.14 regression to avoid having others spend time on tracking this down
meanwhile? (Adding Thorsten on CC.)

Johan