On 11/5/18 6:50 PM, Linus Torvalds wrote: > On Sun, Nov 4, 2018 at 9:08 PM kernel test robot <rong.a.c...@intel.com> > wrote: >> >> FYI, we noticed a -64.1% regression of will-it-scale.per_thread_ops >> due to commit 9bc8039e715d ("mm: brk: downgrade mmap_sem to read when >> shrinking") > > Ugh. That looks pretty bad. > >> in testcase: will-it-scale >> on test machine: 8 threads Ivy Bridge with 16G memory >> with following parameters: >> >> nr_task: 100% >> mode: thread >> test: brk1 >> ucode: 0x20 >> cpufreq_governor: performance > > The reason seems to be way more scheduler time due to lots more > context switches: > >> 34925294 ± 18% +270.3% 1.293e+08 ± 4% >> will-it-scale.time.voluntary_context_switches
And what about this: 0.83 ± 27% +25.9 26.75 ± 11% perf-profile.calltrace.cycles-pp.intel_idle.cpuidle_enter_state.do_idle.cpu_startup_entry.start_secondary 1.09 ± 32% +30.9 31.97 ± 10% perf-profile.calltrace.cycles-pp.cpuidle_enter_state.do_idle.cpu_startup_entry.start_secondary.secondary_startup_64 1.62 ± 36% +44.4 46.01 ± 9% perf-profile.calltrace.cycles-pp.do_idle.cpu_startup_entry.start_secondary.secondary_startup_64 1.63 ± 36% +44.5 46.18 ± 9% perf-profile.calltrace.cycles-pp.cpu_startup_entry.start_secondary.secondary_startup_64 1.63 ± 36% +44.6 46.21 ± 9% perf-profile.calltrace.cycles-pp.start_secondary.secondary_startup_64 1.73 ± 29% +51.1 52.86 ± 2% perf-profile.calltrace.cycles-pp.secondary_startup_64 And the graphs showing less user/kernel time and less "percent_of_cpu_this_job_got"... I didn't spot an obvious mistake in the patch itself, so it looks like some bad interaction between scheduler and the mmap downgrade? > Yang Shi, would you mind taking a look at what's going on? > > Linus >