Johan Hovold reported a heavy performance regression caused by lockdep
cross-release:

 > 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:
 >
 >	28a903f6 ("locking/lockdep: Handle non(or multi)-acquisition
 >	               of a crosslock")
 >
 > which I've verified is the commit which doubled the boot time (compared
 > to 28a903f6^) (added by lockdep crossrelease series [1]).

Currently cross-release performs unwind on every acquisition, but that
is very expensive.

This patch makes unwind optional and disables it by default and only
records acquire_ip.

Full stack traces are sometimes required for full analysis, in which
case a boot paramter, crossrelease_fullstack, can be specified.

On my qemu Ubuntu machine (x86_64, 4 cores, 512M), the regression was
fixed. We measure boot times with 'perf stat --null --repeat 10 $QEMU',
where $QEMU launches a kernel with init=/bin/true:

1. No lockdep enabled:

 Performance counter stats for 'qemu_booting_time.sh bzImage' (10 runs):

       2.756558155 seconds time elapsed                    ( +-  0.09% )

2. Lockdep enabled:

 Performance counter stats for 'qemu_booting_time.sh bzImage' (10 runs):

       2.968710420 seconds time elapsed                    ( +-  0.12% )

3. Lockdep enabled + cross-release enabled:

 Performance counter stats for 'qemu_booting_time.sh bzImage' (10 runs):

       3.153839636 seconds time elapsed                    ( +-  0.31% )

4. Lockdep enabled + cross-release enabled + this patch applied:

 Performance counter stats for 'qemu_booting_time.sh bzImage' (10 runs):

       2.963669551 seconds time elapsed                    ( +-  0.11% )

I.e. lockdep cross-release performance is now indistinguishable from
vanilla lockdep.
Bisected-by: Johan Hovold <johan@kernel.org>
Analyzed-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: amir73il@gmail.com
Cc: axboe@kernel.dk
Cc: darrick.wong@oracle.com
Cc: david@fromorbit.com
Cc: hch@infradead.org
Cc: idryomov@gmail.com
Cc: johannes.berg@intel.com
Cc: kernel-team@lge.com
Cc: linux-block@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-xfs@vger.kernel.org
Cc: oleg@redhat.com
Cc: tj@kernel.org
Link: http://lkml.kernel.org/r/1508921765-15396-5-git-send-email-byungchul.park@lge.comSigned-off-by: Ingo Molnar <mingo@kernel.org>