linux-kernel - Re: MCE, AMD: Hide smp-only code around CONFIG

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Message-ID: <20120211140737.GA26637@elte.hu>
Date:	Sat, 11 Feb 2012 15:07:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kevin Winchester <kjwinchester@...il.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Nick Bowler <nbowler@...iptictech.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: MCE, AMD: Hide smp-only code around CONFIG_SMP


* Kevin Winchester <kjwinchester@...il.com> wrote:

> On 9 February 2012 04:06, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Borislav Petkov <bp@...en8.de> wrote:
> >
> >> +++ b/arch/x86/include/asm/smp.h
> >> @@ -33,8 +33,15 @@ static inline bool cpu_has_ht_siblings(void)
> >>
> >>  DECLARE_PER_CPU(cpumask_var_t, cpu_sibling_map);
> >>  DECLARE_PER_CPU(cpumask_var_t, cpu_core_map);
> >> -/* cpus sharing the last level cache: */
> >> +
> >> +#ifdef CONFIG_SMP
> >> +/* CPUs sharing the last level cache: */
> >>  DECLARE_PER_CPU(cpumask_var_t, cpu_llc_shared_map);
> >> +#else
> >> +static DECLARE_BITMAP(cpu_llc_shared_bits, NR_CPUS) __read_mostly = { [0] = 1UL };
> >> +static struct cpumask *const cpu_llc_shared_map = to_cpumask(cpu_llc_shared_bits);
> >> +#endif
> >
> > Why not just expose it like on SMP?
> >
> > We want to *reduce* the specialness of UP, not increase it - one
> > more word of .data and .text does not matter much - UP is
> > becoming more and more an oddball, rarely tested config. By the
> > time these changes hit any real boxes it will be even more
> > oddball.
> >
> 
> It seems that cpu_llc_shared_map is actually defined in 
> arch/x86/kernel/smpboot.c, which is not compiled/linked for UP 
> builds.
>  Is there an equivalent file for UP that could be used 
> instead, or could the:
> 
> DEFINE_PER_CPU(cpumask_var_t, cpu_llc_shared_map);
> 
> be moved to some other file?

Yes, it should be moved into struct cpuinfo_x86, and thus we'd 
remove cpu_llc_shared_map altogether, it would be named 
cpu->llc_shared_map or so - taking up a single bit (or maybe 
zero bits) on UP.

> Generally, it sounds like you might approve of an eventual 
> merging of the boot paths for SMP and UP.  Is that true?  I 
> wonder how much work that would be.  That would really reduce 
> the specialness of UP.

I generally approve just about any patch that works and reduces 
complexity! :-) The boot path is rather ambitious, but if you 
want to try, feel free ...

Thanks,

	Ingo
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