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

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Date:	Mon, 30 Jul 2012 10:09:53 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	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.

On Mon, 2012-07-30 at 11:40 +0930, Rusty Russell 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>

Acked-by: Suresh Siddha <suresh.b.siddha@...el.com>

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