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

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

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

Whew, I still get confused about who to Cc, thanks for adding them.
and I also didn't include the percpu memory allocator maintainers, who may
have opinion. let's add them too.

> 
> > ---
> >  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
> 

-- 
Thanks,
Hyeonggon