linux-kernel - Re: [PATCH v2] x86: don't ever patch back to UP if we unplug cpus.

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Date:	Wed, 22 Aug 2012 11:41:53 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paul.mckenney@...ibm.com>
Subject: Re: [PATCH v2] x86: don't ever patch back to UP if we unplug cpus.


* Rusty Russell <rusty@...tcorp.com.au> wrote:

> We still patch SMP instructions to UP variants if we boot with a
> single CPU, but not at any other time.  In particular, not if we
> unplug CPUs to return to a single cpu.
> 
> Paul McKenney points out:
> 
>  mean offline overhead is 6251/48=130.2 milliseconds.
> 
>  If I remove the alternatives_smp_switch() from the offline
>  path [...] the mean offline overhead is 550/42=13.1 milliseconds
> 
> Basically, we're never going to get those 120ms back, and the code is
> pretty messy.
> 
> We get rid of:
> 1) The "smp-alt-once" boot option.  It's actually "smp-alt-boot", the
>    documentation is wrong.  It's now the default.
> 2) The skip_smp_alternatives flag used by suspend.
> 3) arch_disable_nonboot_cpus_begin() and arch_disable_nonboot_cpus_end()
>    which were only used to set this one flag.
> 
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> ---
>  Documentation/kernel-parameters.txt |    3 -
>  arch/x86/include/asm/alternative.h  |    4 -
>  arch/x86/kernel/alternative.c       |  104 +++++++-----------------------------
>  arch/x86/kernel/smpboot.c           |   20 ------
>  kernel/cpu.c                        |   11 ---
>  5 files changed, 27 insertions(+), 115 deletions(-)

and this version breaks the build on allyesconfig-x86-64:

arch/x86/xen/smp.c:380:3: error: implicit declaration of function ‘alternatives_smp_switch’ [-Werror=implicit-function-declaration]

Thanks,

	Ingo
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