Re: [PATCH v7 5/5] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  3 Feb 2023 17:18:37 +1000 Nicholas Piggin <npiggin@gmail.com> wrote:

> On a 16-socket 192-core POWER8 system, the context_switch1_threads
> benchmark from will-it-scale (see earlier changelog), upstream can
> achieve a rate of about 1 million context switches per second, due to
> contention on the mm refcount.
> 
> 64s meets the prerequisites for CONFIG_MMU_LAZY_TLB_SHOOTDOWN, so enable
> the option. This increases the above benchmark to 118 million context
> switches per second.

Is that the best you can do ;)

> This generates 314 additional IPI interrupts on a 144 CPU system doing
> a kernel compile, which is in the noise in terms of kernel cycles.
> 
> ...
>
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -265,6 +265,7 @@ config PPC
>  	select MMU_GATHER_PAGE_SIZE
>  	select MMU_GATHER_RCU_TABLE_FREE
>  	select MMU_GATHER_MERGE_VMAS
> +	select MMU_LAZY_TLB_SHOOTDOWN		if PPC_BOOK3S_64
>  	select MODULES_USE_ELF_RELA
>  	select NEED_DMA_MAP_STATE		if PPC64 || NOT_COHERENT_CACHE
>  	select NEED_PER_CPU_EMBED_FIRST_CHUNK	if PPC64

Can we please have a summary of which other architectures might benefit
from this, and what must they do?

As this is powerpc-only, I expect it won't get a lot of testing in
mm.git or in linux-next.  The powerpc maintainers might choose to merge
in the mm-stable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm if this is a
concern.