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From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Matthew Wilcox <willy@infradead.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default
Date:   Sat, 21 Jan 2023 12:39:42 +0900
Message-Id: <20230121033942.350387-1-42.hyeyoo@gmail.com>
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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