linux-kernel - Re: [tip:locking/urgent] locking/lockdep: Disable cross-release features for now

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Date:   Tue, 17 Oct 2017 18:21:24 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Byungchul Park <byungchul.park@....com>, johan@...nel.org,
        arnd@...db.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org, hpa@...or.com,
        tony@...mide.com, linux-tip-commits@...r.kernel.org,
        kernel-team@....com
Subject: Re: [tip:locking/urgent] locking/lockdep: Disable cross-release
 features for now


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Tue, 17 Oct 2017, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@...utronix.de> wrote:
> > > On Tue, 17 Oct 2017, Ingo Molnar wrote:
> > > > No, please fix performance.
> > > 
> > > You know very well that with the cross release stuff we have to take the
> > > performance hit of stack unwinding because we have no idea whether there
> > > will show up a new lock relation later or not. And there is not much you
> > > can do in that respect.
> > > 
> > > OTOH, the cross release feature unearthed real deadlocks already so it is a
> > > valuable debug feature and having an explicit config switch which defaults
> > > to N is well worth it.
> > 
> > I disagree, because even if that's correct, the choices are not binary. The 
> > performance regression was a slowdown of around 7x: lockdep boot overhead on that 
> > particula system went from +3 seconds to +21 seconds...
> 
> Hmm, I might have missed something, but what I've seen in this thread is:
> 
> > > > 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.
> 
> So that's 2x not 7x. [...]

Yeah, so what you missed I think is that the no-lockdep bootup time is 14 seconds. 
So we have:

   vanilla:                        14 secs
   lockdep:                        17 secs (+3 secs)
   lockdep+crossrelease:           35 secs (+21 secs)

So lockdep overhead got 7x worse on this system.

Thanks,

	Ingo