Message-ID: <4A39641D.2090201@tlinx.org>
Date: Wed, 17 Jun 2009 14:46:05 -0700
From: Linda Walsh <lkml@tlinx.org>
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Subject: IGNORE: Re: non-standard config in my running distro Default kernel:
 Effect?
References: <4A395F80.7060504@tlinx.org>
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Please ignore this non-issue.  I should have sorted my
compare files.  (Sigh)...

Linda Walsh wrote:
>
> I found what appears to be a "non-standard configuration" in a
> distro's "Default-SMP", non-Xen kernel, where /proc/config.gz
> from the running kernel has (I only ran into this doing a diff between
> a manually constructed config and my running kernel):
>
>    CONFIG_X86_SMP=y
>    CONFIG_X86_64_SMP=y
>
> But does NOT have:
>
>    CONFIG_USE_GENERIC_SMP_HELPERS=y
>
> From what I can tell, this shouldn't be a possible configuration, since
> CONFIG_X86_SMP should force it to be set to 'y' (not unset).
>
> Would this config, (SMP='y', generic_smp_helpers=(undef) ) have any
> odd effects?  Is there a specific "effect" or "feature" one gets
> by having 'smp=y' but not having the smp_helpers enabled?
>
> In my x86_64 arch, the places I see it tested are:
>
> block/blk-softirq.c:
>    #if defined(CONFIG_SMP) && defined(CONFIG_USE_GENERIC_SMP_HELPERS)
>    #else /* CONFIG_SMP && CONFIG_USE_GENERIC_SMP_HELPERS */
> block/blk-sysfs.c:
>    #if defined(CONFIG_USE_GENERIC_SMP_HELPERS)
> include/linux/smp.h:
>    #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
> /kernel/Kconfig.hz:
>    def_bool HIGH_RES_TIMERS && (!SMP || USE_GENERIC_SMP_HELPERS)
> kernel/softirq.c:
>    #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
>    #else /* CONFIG_USE_GENERIC_SMP_HELPERS */
>
> ---
> It appears it may disable some softirq routines.
>
> In include/linux/smp.h, having it undefined would disable the 
> definition of the functions:
>  generic_smp_call_function_single_interrupt,
>  generic_smp_call_function_interrupt,
>  ipi_call_lock,      ipi_call_unlock,
>  ipi_call_lock_irq & ipi_call_unlock_irq
>
> Does this have any performance or integrity (lost interrupt)
> implications?
>
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