On Sat, Jan 21, 2023 at 12:29:44PM +0100, Vlastimil Babka wrote: > 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. Whew, I still get confused about who to Cc, thanks for adding them. and I also didn't include the percpu memory allocator maintainers, who may have opinion. let's add them too. > > > --- > > 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 > -- Thanks, Hyeonggon
