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

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Date:	Thu, 9 Feb 2012 20:00:45 -0400
From:	Kevin Winchester <kjwinchester@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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?

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.

-- 
Kevin Winchester
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