Re: [PATCH 0/5] sched: Make CONFIG_SCHED_DEBUG features unconditional

[Ingo Molnar]

* Valentin Schneider <vschneid@xxxxxxxxxx> wrote:

> On 17/03/25 11:42, 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.
> >
> > 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 ...
> >
> 
> Pretty much every distro I'm aware of has CONFIG_SCHED_DEBUG=y; a quick check
> tells me it's been like so for RHEL since at least 2013, and that's from a
> commit copying configs from RHEL-6 to RHEL-7.
> 
> Two things however come to mind:
> 
> 1) What does this mean for the debug stuff we've repeatedly said wasn't ABI
>    because it was under CONFIG_SCHED_DEBUG? I've been burned by making
>    sched_domain.flags read-only, and there's still writable stuff:
> 
>    # ls -al /sys/kernel/debug/sched/domains/cpu0/domain0/
>    total 0
>    drwxr-xr-x. 2 root root 0 Mar 19 04:36 .
>    drwxr-xr-x. 3 root root 0 Mar 19 04:36 ..
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 busy_factor
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 cache_nice_tries
>    -r--r--r--. 1 root root 0 Mar 19 04:36 flags
>    -r--r--r--. 1 root root 0 Mar 19 04:36 groups_flags
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 imbalance_pct
>    -r--r--r--. 1 root root 0 Mar 19 04:36 level
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 max_interval
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 max_newidle_lb_cost
>    -rw-r--r--. 1 root root 0 Mar 19 04:36 min_interval
>    -r--r--r--. 1 root root 0 Mar 19 04:36 name
> 
>    + all the non topology related debug knobs.

Yeah, I don't think these or other sysctls are as contentious as 
previously thought. We might want to put '/debug/' into the directory 
name above, or we could move it over to debugfs entirely - but we 
should make it clear via the name that these are debugging knobs in 
essence.

> 2) Peter mentioned a few times that, last time it was benchmarked, there
>    were noticeable perf differences between CONFIG_SCHED_DEBUG=n and
>    CONFIG_SCHED_DEBUG=y. This would be an occasion to re-measure that and
>    potentially move (some of) these checks to e.g. a sched_debug_verbose
>    static key.

Yeah, and this is an argument strongly *in favor* of eliminating 
CONFIG_SCHED_DEBUG: in a way the CONFIG_SCHED_DEBUG "option" created a 
false sense of "it's only debug code". But it's not a genuine debug 
option, it's actual overhead for the vast majority of Linux distros and 
users.

So let's just eliminate SCHED_DEBUG, and fix any overhead. It's exactly 
what we should do anyway - nothing changes IMHO, just the appearance of 
urgency. :-)

Thanks,

	Ingo