Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default

[Vlastimil Babka]

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%*
> 
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>

Looks like it's there since the beginning of preempt and pre-git. But
probably should be something for scheduler maintainers rather than mm/slab,
even if the impact manifests there. You did Cc Ingo (the original author) so
let me Cc the rest here.

> ---
>  lib/Kconfig.debug | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index ddbfac2adf9c..f6f845a4b9ec 100644
> --- a/lib/Kconfig.debug
> +++ b/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