linux-kernel - Re: [PATCH 1/2] x86: Track minimum microcode revision globally v2

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Date:	Wed, 13 Jun 2012 07:49:07 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andi Kleen <andi@...stfloor.org>, x86@...nel.org, hpa@...or.com,
	eranian@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86: Track minimum microcode revision globally v2

On Wed, Jun 13, 2012 at 10:49:52AM +0200, Peter Zijlstra wrote:
> On Tue, 2012-06-12 at 14:25 -0700, Andi Kleen wrote:
> > +void update_min_microcode(struct cpuinfo_x86 *c)
> > +{
> > +       int i;
> > +
> > +       for_each_online_cpu (i)
> 
> Superfluous whitespace
> 
> > +               if (cpu_data(i).microcode < c->microcode)
> > +                       return;
> 
> That needs {}

You must be following a different code style guide than the Linux one.

> >         c->microcode = val[1];
> >  
> > +       update_min_microcode(c);
> >         return 0;
> >  }
> 
> Doing it here means doing the for_each_cpu thing with preempt/irqs
> disabled, that's not funny.

Well a ucode update is a really slow operation anyways.  And the loop
gets stopped at the first mismatch. So it'll never be n^2 and in most
cases much faster. Generally the loop should be several orders 
of magnitude less than the actual cost of the update, even on large 
systems.

> Also this is still a O(n^2) proposition.. so how is this better than the
> notifier thing I had?

Simpler at least. 

I don't know why people love notifiers, they are a "COME FROM" and make
every code who uses them a mess. 

As for CPU_STARTING don't know how complicated it would be. I suppose
it could be done as a follow up.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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