Re: + lib-kconfigdebug-do-not-enable-debug_preempt-by-default.patch added to mm-nonmm-unstable branch

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Jan 26, 2023 at 03:23:39PM -0800, Andrew Morton wrote:
> 
> The patch titled
>      Subject: lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default
> has been added to the -mm mm-nonmm-unstable branch.  Its filename is
>      lib-kconfigdebug-do-not-enable-debug_preempt-by-default.patch
> 
> This patch will shortly appear at
>      https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-kconfigdebug-do-not-enable-debug_preempt-by-default.patch
> 
> This patch will later appear in the mm-nonmm-unstable branch at
>     git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> 
> Before you just go and hit "reply", please:
>    a) Consider who else should be cc'ed
>    b) Prefer to cc a suitable mailing list as well
>    c) Ideally: find the original patch on the mailing list and do a
>       reply-to-all to that, adding suitable additional cc's
> 
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
> 
> The -mm tree is included into linux-next via the mm-everything
> branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> and is updated there every 2-3 working days
> 
> ------------------------------------------------------
> From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Subject: lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default
> Date: Sat, 21 Jan 2023 12:39:42 +0900
> 
> 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%*
> 
> Link: https://lkml.kernel.org/r/20230121033942.350387-1-42.hyeyoo@gmail.com
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dennis Zhou <dennis@kernel.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>

Thanks for picking this up!

but looks like missing Mel's Acked-by. could you add that please?

> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  lib/Kconfig.debug |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> --- a/lib/Kconfig.debug~lib-kconfigdebug-do-not-enable-debug_preempt-by-default
> +++ a/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
> _
> 
> Patches currently in -mm which might be from 42.hyeyoo@gmail.com are
> 
> mm-page_owner-record-single-timestamp-value-for-high-order-allocations.patch
> lib-kconfigdebug-do-not-enable-debug_preempt-by-default.patch
>