Date:   Wed, 1 Feb 2023 19:09:55 -0800
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     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>,
        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>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable
 DEBUG_PREEMPT by default
Message-ID: <20230202030955.nfejzcgn6edp6cgl@offworld>
Mail-Followup-To: Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        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>,
        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>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230121033942.350387-1-42.hyeyoo@gmail.com>
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On Sat, 21 Jan 2023, 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>

Acked-by: Davidlohr Bueso <dave@stgolabs.net>