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SubjectRe: [PATCH v2] x86: don't ever patch back to UP if we unplug cpus.

* Rusty Russell <rusty@rustcorp.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@rustcorp.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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