Re: [-v2 PATCH 2/6] powernow-k8: Add core performance boost support

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 31 Mar 2010 08:13:43 +0200 Borislav Petkov <bp@amd64.org> wrote:

> > > +static void _cpb_toggle_msrs(bool t)
> > > +{
> > > +	int cpu;
> > > +
> > > +	rdmsr_on_cpus(cpu_online_mask, MSR_K7_HWCR, msrs);
> > > +
> > > +	for_each_cpu(cpu, cpu_online_mask) {
> > > +		struct msr *reg = per_cpu_ptr(msrs, cpu);
> > > +		if (t)
> > > +			reg->l &= ~BIT(25);
> > > +		else
> > > +			reg->l |= BIT(25);
> > > +	}
> > > +	wrmsr_on_cpus(cpu_online_mask, MSR_K7_HWCR, msrs);
> > > +}
> > 
> > What prevents cpu_online_mask from changing while this is happening?
> 
> Frankly, nothing.

Sneak a get_online_cpus()/put_online_cpus() in there?

> And yes, we talked a lot about this internally, since,
> you need this bit cleared on _all_ cores for the processor to boost.
> Now, if you've offlined some of the cores, you won't be able to execute
> any code on them and thus clear this bit.
> 
> However, this interface is there only for benchmarking purposes and
> stuff, i.e. normally, you shouldn't need to touch it _at_ _all_ and
> boosting will work out of the box and without user interaction.
> 
> So, IMHO, hotplug notifiers won't work since you need all cores online
> at that particular moment to enable boosting. It sounds more reasonable
> to me to WARN when a core is missing from the mask that boosting cannot
> be enabled. I'll cook up something later today.