linux-kernel - Re: [PATCH 0/5] sched: Make CONFIG_SCHED

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Message-ID: <Z9szt3MpQmQ56TRd@gmail.com>
Date: Wed, 19 Mar 2025 22:14:31 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 0/5] sched: Make CONFIG_SCHED_DEBUG features unconditional


* Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:

> 
> 
> On 3/17/25 16:12, Ingo Molnar wrote:
> > For more than a decade, CONFIG_SCHED_DEBUG=y has been enabled
> > in all the major Linux distributions:
> > 
> >     /boot/config-6.11.0-19-generic:CONFIG_SCHED_DEBUG=y
> > 
> > The reason is that while originally CONFIG_SCHED_DEBUG started
> > out as a debugging feature, over the years (decades ...) it has
> > grown various bits of statistics, instrumentation and
> > control knobs that are useful for sysadmin and general software
> > development purposes as well.
> 
> A tunable like base_slice which is the only tunable available for EEVDF is under the debug.
> So an option is to get rid of CONFIG_SCHED_DEBUG and make it available to all.
> 
> We had seen performance regression when domains folder was built with cpu hotplug.
> Later that was moved iff verbose was enabled. Maybe something like that can be done
> if something is hurting performance.
> 
> > 
> > But within the kernel we still pretend that there's a choice,
> > and sometimes code that is seemingly 'debug only' creates overhead
> > that should be optimized in reality.
> > 
> > So make it all official and make CONFIG_SCHED_DEBUG unconditional.
> > This gets rid of a large amount of #ifdefs, so good riddance ...
> > 
> 
> There are some references in selftest like these, maybe remove them as well?
> 
> tools/testing/selftests/sched_ext/config:CONFIG_SCHED_DEBUG=y
> tools/testing/selftests/sched/config:CONFIG_SCHED_DEBUG=y
> tools/testing/selftests/wireguard/qemu/debug.config:CONFIG_SCHED_DEBUG=y

Indeed - fixed.

I left out all the defconfigs from the patches, because there's a lot 
of them (~79 reference CONFIG_SCHED_DEBUG ...) and they get refreshed 
naturally in any case.

> Also ran unixbench and hackbench on 80 CPU system (1NUMA) with and 
> without CONFIG_SCHED_DEBUG. hackbench numbers are almost the same.
>
> for unixbench, process creation/Context Switching show 1-2% 
> improvement with CONFIG_SCHED_DEBUG=n

Thank you for the testing! I'll add:

  Tested-by: Shrikanth Hegde <sshegde@...ux.ibm.com>

to the series if you don't mind.

And irrespectively of this series we should probably look at that 1-2% 
overhead in unixbench context switching overhead, maybe there's a few 
low hanging fruits in the debug code.

	Ingo