From: Greg Kurz <gkurz@linux•vnet.ibm.com>
To: Scott Wood <scottwood@freescale•com>
Cc: Michael Ellerman <michael@ellerman•id.au>, linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH 2 1/4] powerpc: drop the ability to tweak SMT mode at boot time
Date: Mon, 8 Dec 2014 09:23:21 +0100 [thread overview]
Message-ID: <20141208092321.673a0be3@bahia.local> (raw)
In-Reply-To: <1417805565.334.15.camel@freescale.com>
On Fri, 5 Dec 2014 12:52:45 -0600
Scott Wood <scottwood@freescale•com> wrote:
> On Fri, 2014-12-05 at 16:14 +0100, Greg Kurz wrote:
> > The smt-enabled kernel parameter basically leaves unwanted cpus executing
> > in firmware or wherever they happen to be. The very same applies to the
> > ibm,smt-enabled DT property which is no more used by anything known. These
> > are hacks that shoudn't be used in a production environment.
> >
> > Quoting mpe, "there are better ways for firmware to disable SMT".
>
Hi Scott,
> Those "better ways" don't apply to Freescale chips, where the OS enables
> (or not) SMT without any interaction with firmware. I don't care about
> the ibm,smt-enabled property, but can we please keep the smt-enabled
> boot option?
>
Fair enough for the firmware side, what about CPU hot(un)plug then ?
> > It also has an evil side effect on the split-core feature for powernv. The
> > code needs all the cpus to participate to the split mode update: it relies
> > on smp_send_reschedule() to get offline ones to do so. This doesn't work with
> > cpus that haven't come up... The consequence is a kernel hang on powernv when
> > trying to limit the number of hw threads at boot time (e.g. smt-enabled to
> > anything but 8 on POWER8).
>
> In that case could you disable the option only on that hardware?
>
The fact it breaks only powernv doesn't mean it is a powernv only issue.
The smt-enabled feature is a hack because it leaves some cpus in a undefined
state from a kernel POV. Moreover it drags about 80 lines of code and sits
entirely in common ppc64 code. I would reverse the question then ? Why not
moving smt-enabled code to freescale only ?
> > This patch simply removes both the smt-enabled kernel parameter and the
> > ibm,smt-enabled property for all platforms. The new default is to start
> > all hw threads. That leaves /sys the only supported API to change SMT
> > settings.
>
> How would you use /sys for this? Are you talking about CPU hotplug?
>
Yes. This is the safer way to offline cpus.
> -Scott
>
Cheers.
--
Greg
next prev parent reply other threads:[~2014-12-08 8:23 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-05 15:13 [PATCH 2 0/4] powerpc: don't mess with SMT at boot time Greg Kurz
2014-12-05 15:14 ` [PATCH 2 1/4] powerpc: drop the ability to tweak SMT mode " Greg Kurz
2014-12-05 18:52 ` Scott Wood
2014-12-08 8:23 ` Greg Kurz [this message]
2014-12-08 22:39 ` Scott Wood
2014-12-09 4:11 ` Michael Ellerman
2014-12-09 8:53 ` Greg Kurz
2014-12-09 21:04 ` Scott Wood
2014-12-09 23:56 ` Michael Ellerman
2014-12-10 0:14 ` Scott Wood
2014-12-10 2:14 ` Michael Ellerman
2014-12-10 23:50 ` Scott Wood
2014-12-12 5:19 ` Michael Ellerman
2014-12-05 15:14 ` [PATCH 2 2/4] powerpc: drop smt_enabled_at_boot Greg Kurz
2014-12-05 15:15 ` [PATCH 2 3/4] powerpc: drop smp_generic_cpu_bootable() Greg Kurz
2014-12-05 15:34 ` Greg Kurz
2014-12-05 15:15 ` [PATCH 2 4/4] powerpc: drop the cpu_bootable hook Greg Kurz
2014-12-05 15:42 ` [PATCH] " Greg Kurz
2014-12-12 5:22 ` [PATCH 2 0/4] powerpc: don't mess with SMT at boot time Michael Ellerman
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