Cannot pick up this series yet, but I have enhanced the changelog to: =============> Subject: locking/lockdep: Introduce CONFIG_CROSSRELEASE_STACK_TRACE and make it not unwind by default From: Byungchul Park <byungchul.p...@lge.com> Date: Tue, 24 Oct 2017 18:38:03 +0900
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: > > 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]). Currently crossrelease 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 CROSSRELEASE_STACK_TRACE can be enabled. 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 + crossrelease enabled: Performance counter stats for 'qemu_booting_time.sh bzImage' (10 runs): 3.153839636 seconds time elapsed ( +- 0.31% ) 4. Lockdep enabled + crossrelease 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-crossrelease performance is now indistinguishable from vanilla lockdep.