linux-kernel - Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251014094210.GQ3245006@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 11:42:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	thomas.weissschuh@...utronix.de, Li Chen <chenl311@...natelecom.cn>,
	Bibo Mao <maobibo@...ngson.cn>, Mete Durlu <meted@...ux.ibm.com>,
	Tobias Huschle <huschle@...ux.ibm.com>,
	Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>,
	Guo Weikang <guoweikang.kernel@...il.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>,
	Swapnil Sapkal <swapnil.sapkal@....com>,
	"Yury Norov [NVIDIA]" <yury.norov@...il.com>,
	Sudeep Holla <sudeep.holla@....com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Andrea Righi <arighi@...dia.com>,
	Yicong Yang <yangyicong@...ilicon.com>,
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED_MC to guard
 MC scheduling bits

On Tue, Oct 14, 2025 at 11:25:53AM +0200, Geert Uytterhoeven wrote:
> Hoi Peter,
> 
> On Thu, 28 Aug 2025 at 14:57, Peter Zijlstra <peterz@...radead.org> wrote:
> > Now, when I look at unifying those config options (there's a metric ton
> > of crap that's duplicated in the arch/*/Kconfig), I end up with something
> > like the below.
> >
> > And while that isn't exact, it is the closest I could make it without
> > making a giant mess of things.
> >
> > WDYT?
> 
> Thanks for your patch, which is now commit 7bd291abe2da09f5 ("sched:
> Unify the SCHED_{SMT,CLUSTER,MC} Kconfig") in v6.18-rc1.
> 
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -41,6 +41,44 @@ config HOTPLUG_SMT
> >  config SMT_NUM_THREADS_DYNAMIC
> >         bool
> >
> > +config ARCH_SUPPORTS_SCHED_SMT
> > +       bool
> > +
> > +config ARCH_SUPPORTS_SCHED_CLUSTER
> > +       bool
> > +
> > +config ARCH_SUPPORTS_SCHED_MC
> > +       bool
> > +
> > +config SCHED_SMT
> > +       bool "SMT (Hyperthreading) scheduler support"
> > +       depends on ARCH_SUPPORTS_SCHED_SMT
> > +       default y
> 
> This is now enabled by default everywhere, while it was disabled by
> default on most architectures before...

I'm not sure ARCH_SUPPORTS_SCHED_SMT counts as everywhere, but yes.
A fair deal of the architectures had all this default yes, and I had to
pick something. Can't make an omelette without breaking an egg and all
that :/

> > +       help
> > +         Improves the CPU scheduler's decision making when dealing with
> > +         MultiThreading at a cost of slightly increased overhead in some
> > +         places. If unsure say N here.
> 
> So it should default to n?

That's just help text that got carried around. Many of the architectures
that had default y still had this text on. I suppose we can change it if
someone cares.

> If it is really needed on some architectures or platforms, I guess
> they can still select it explicitly?

There were 4 cases:

 - arch doesn't support SMT
 - arch supports SMT and lets user pick, default Y
 - arch supports SMT and lets user pick, default N
 - arch mandates SMT

Of those 3 are still possible, the one we lost is the default N case.

Old configs that have =N will continue to have N. New configs might end
up with Y.

Why is this a problem?