linux-kernel - Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG

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Date:   Mon, 23 Jan 2023 09:58:30 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable
 DEBUG_PREEMPT by default

On Sat 21-01-23 20:54:15, Hyeonggon Yoo wrote:
> On Sat, Jan 21, 2023 at 12:29:44PM +0100, Vlastimil Babka wrote:
> > 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%*

Do you happen to have any perf data collected during those runs? I
would be interested in the memcg side of things. Maybe we can do
something better there.
-- 
Michal Hocko
SUSE Labs