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Subject[PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default
Date
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@gmail.com>
---
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
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