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

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Date:   Wed, 25 Jan 2023 18:02:04 -0800
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
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 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.

Probably not that big for real workloads, but still nice to have.

Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thank you!