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Sat, 21 Jan 2023 11:29:44 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id KKH1M6jMy2O0EgAAMHmgww (envelope-from ); Sat, 21 Jan 2023 11:29:44 +0000 Message-ID: <86e68d3b-b029-5e82-5bbc-e0ccc2ae1d36@suse.cz> Date: Sat, 21 Jan 2023 12:29:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default To: Hyeonggon Yoo <42.hyeyoo@gmail.com>, Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Roman Gushchin Cc: Ingo Molnar , Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider References: <20230121033942.350387-1-42.hyeyoo@gmail.com> Content-Language: en-US From: Vlastimil Babka In-Reply-To: <20230121033942.350387-1-42.hyeyoo@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E9C071C0006 X-Stat-Signature: it6an8xiib3o5kkj6x7egrpn51w6se4p X-Rspam-User: X-HE-Tag: 1674300586-442117 X-HE-Meta: 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 Uetrq7qu 1vICOPEYCuWQNXG659UvgH/FCpcPKLxqfHmrCfxNwvXcx+X5D6xHAAUiejCy+Aan46GlzMYEZDXnRBSBCYJBLYqhcaNeFjOsIxktPvc/A04Bg7VrXdWdYDUBeF38fXyUEPZ7CgC2OIJmsX4alxXAKw+6qEB7wa2MHJVpoV6/T1EH1BJpR1nPKBUc7EZo12CxFV+SFx0F/GVhIXsWWLMNlqdvwonqF+jYlt3Dz14Um3S/q91FPfr19UKHvhkWk+D4lxF1w+Y8qttmb/HM0RkHdk0mSaiTRENbaZmDI5tchFuMmjA/b8FJrXBOhuJPzLh6Qn7ZsGkQq56ZMY9c3uW/z6TVrsU0jnZiQMTK48kTBTUqQkxg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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@gmail.com> 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. > --- > 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