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

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Date:   Fri, 27 Jan 2023 20:45:28 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     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>,
        Vlastimil Babka <vbabka@...e.cz>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.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
Subject: Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable
 DEBUG_PREEMPT by default

On Wed, Jan 25, 2023 at 06:02:04PM -0800, Roman Gushchin wrote:
> On Sat, Jan 21, 2023 at 12:39:42PM +0900, 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@...il.com>
> 
> Nice!
> 
> I checkout my very simple kmem performance test (1M allocations 8-bytes allocations)
> and it shows ~30% difference: 112319 us with vs 80836 us without.

Hello Roman,

Oh, it has higher impact on micro benchmark.

> 
> Probably not that big for real workloads, but still nice to have.
> 
> Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thank you for kindly measuring impact of this patch
and giving ack!

> Thank you!
> 

--
Thanks,
Hyeonggon