lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default

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%*

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hyeonggon Yoo <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Cc: Ben Segall <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Steven Rostedt (Google) <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: hygoni

lib/Kconfig.debug

@@ -1246,13 +1246,16 @@ 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