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

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Feb 26, 2023 at 02:12:38PM -0800, Andrew Morton wrote:
> 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.

I haven't really had time to page all of this back in, but x86 is very
close to be able to use this, it mostly just needs cleaning up some
accidental active_mm usage.

I've got a branch here:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/lazy

That's mostly Nick's patches with a bunch of Andy's old patches stuck on
top. I also have a pile of notes, but alas, not finished in any way.