linux-kernel - Re: [patch] Mixed Madison and Montecito system support

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Message-ID: <20061025164253.A21790@unix-os.sc.intel.com>
Date:	Wed, 25 Oct 2006 16:42:53 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Russ Anderson <rja@....com>
Cc:	"Luck, Tony" <tony.luck@...el.com>, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] Mixed Madison and Montecito system support

On Tue, Oct 24, 2006 at 07:56:45PM -0500, Russ Anderson wrote:
> Tony Luck wrote:
> > 
> > Cc: linux-kernel for generic bit of this change.  Rest of patch was
> > posted to linux-ia64: http://marc.theaimsgroup.com/?l=linux-ia64&m=116070997529216&w=2
> > 
> > On Thu, Oct 12, 2006 at 10:25:58PM -0500, Russ Anderson wrote:
> > >  int sched_create_sysfs_power_savings_entries(struct sysdev_class *cls)
> > >  {
> > > -	int err = 0;
> > > +	int err = 0, c;
> > >  
> > >  #ifdef CONFIG_SCHED_SMT
> > > -	if (smt_capable())
> > > -		err = sysfs_create_file(&cls->kset.kobj,
> > > +	for_each_online_cpu(c)
> > > +		if (smt_capable(c)) {
> > > +			err = sysfs_create_file(&cls->kset.kobj,
> > >  					&attr_sched_smt_power_savings.attr);
> > > +			break;
> > > +		}
> > >  #endif
> > 
> > What if you booted an all-Madison system, and then hot-plugged some
> > Montecitos later?  Either we'd need the hotplug cpu code to run through
> > this routine again to re-test whether any cpu has multi-thread support
> > (it doesn't look like it does that now).
> > 
> > Or perhaps it would be simpler to dispense with this test and always
> > call sysfs_create_file() here (still inside CONFIG_SCHED_SMT) so that
> > the hook is always present to tune the scheduler (even if it may be
> > ineffective on a no-smt system)?
> 
> I like that idea.  Any objections or comments?

I added it so that these entries will not confuse users of a non-smt/mc
systems. But mixed type of processors and cpu hotplug really complicates the
things..

May be a check of something like "is this platform capable of
supporting any multi-core/multi-threaded processor package?" helps..

As there is no well defined mechanism to find out that and for simplicity
reasons, we should probably go with Tony's suggestion.

Russ I can post a patch, removing both smt_capable() and mc_capable()
checks.

Today this sysfs variable is not documented. But when it happens, we
need to clearly document that these variables have no meaning when
the system doesn't have cpus with threads/cores.

thanks,
suresh
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