On 1/21/23 04:39, Hyeonggon Yoo wrote: > In workloads where this_cpu operations are frequently performed, > enabling DEBUG_PREEMPT may result in significant increase in > runtime overhead due to frequent invocation of > __this_cpu_preempt_check() function. > > This can be demonstrated through benchmarks such as hackbench where this > configuration results in a 10% reduction in performance, primarily due to > the added overhead within memcg charging path. > > Therefore, do not to enable DEBUG_PREEMPT by default and make users aware > of its potential impact on performance in some workloads. > > hackbench-process-sockets > debug_preempt no_debug_preempt > Amean 1 0.4743 ( 0.00%) 0.4295 * 9.45%* > Amean 4 1.4191 ( 0.00%) 1.2650 * 10.86%* > Amean 7 2.2677 ( 0.00%) 2.0094 * 11.39%* > Amean 12 3.6821 ( 0.00%) 3.2115 * 12.78%* > Amean 21 6.6752 ( 0.00%) 5.7956 * 13.18%* > Amean 30 9.6646 ( 0.00%) 8.5197 * 11.85%* > Amean 48 15.3363 ( 0.00%) 13.5559 * 11.61%* > Amean 79 24.8603 ( 0.00%) 22.0597 * 11.27%* > Amean 96 30.1240 ( 0.00%) 26.8073 * 11.01%* > > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> Looks like it's there since the beginning of preempt and pre-git. But probably should be something for scheduler maintainers rather than mm/slab, even if the impact manifests there. You did Cc Ingo (the original author) so let me Cc the rest here. > --- > lib/Kconfig.debug | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index ddbfac2adf9c..f6f845a4b9ec 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -1176,13 +1176,16 @@ config DEBUG_TIMEKEEPING > config DEBUG_PREEMPT > bool "Debug preemptible kernel" > depends on DEBUG_KERNEL && PREEMPTION && TRACE_IRQFLAGS_SUPPORT > - default y > help > If you say Y here then the kernel will use a debug variant of the > commonly used smp_processor_id() function and will print warnings > if kernel code uses it in a preemption-unsafe way. Also, the kernel > will detect preemption count underflows. > > + This option has potential to introduce high runtime overhead, > + depending on workload as it triggers debugging routines for each > + this_cpu operation. It should only be used for debugging purposes. > + > menu "Lock Debugging (spinlocks, mutexes, etc...)" > > config LOCK_DEBUGGING_SUPPORT
