Adding Peter to the cc as this should go via the tip tree even though Ingo is cc'd already. Leaving full context and responding inline. On Sat, Jan 21, 2023 at 12:39:42PM +0900, 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> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> This has been default y since very early on in the development of the BKL removal. It was probably selected by default because it was expected there would be a bunch of new SMP-related bugs. These days, there is no real reason to enable it by default except when debugging a preempt-related issue or during development. It's not like CONFIG_SCHED_DEBUG which gets enabled in a lot of distros as it has some features which are useful in production (which is unfortunate but splitting CONFIG_SCHED_DEBUG is a completely separate topic). > --- > 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 > -- > 2.34.1 > > -- Mel Gorman SUSE Labs
