Dramatic lockdep slowdown in 4.14

[Peter Zijlstra]

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.