Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

[Barry Song]

On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>
> On 2022/9/27 14:16, Anshuman Khandual wrote:
> > [...]
> >
> > On 9/21/22 14:13, Yicong Yang wrote:
> >> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> >> +{
> >> +    /* for small systems with small number of CPUs, TLB shootdown is cheap */
> >> +    if (num_online_cpus() <= 4)
> >
> > It would be great to have some more inputs from others, whether 4 (which should
> > to be codified into a macro e.g ARM64_NR_CPU_DEFERRED_TLB, or something similar)
> > is optimal for an wide range of arm64 platforms.
> >

I have tested it on a 4-cpus and 8-cpus machine. but i have no machine
with 5,6,7
cores.
I saw improvement on 8-cpus machines and I found 4-cpus machines don't need
this patch.

so it seems safe to have
if (num_online_cpus()  < 8)

>
> Do you prefer this macro to be static or make it configurable through kconfig then
> different platforms can make choice based on their own situations? It maybe hard to
> test on all the arm64 platforms.

Maybe we can have this default enabled on machines with 8 and more cpus and
provide a tlbflush_batched = on or off to allow users enable or
disable it according
to their hardware and products. Similar example: rodata=on or off.

Hi Anshuman, Will,  Catalin, Andrew,
what do you think about this approach?

BTW, haoxin mentioned another important user scenarios for tlb bach on arm64:
https://lore.kernel.org/lkml/393d6318-aa38-01ed-6ad8-f9eac89bf0fc@xxxxxxxxxxxxxxxxx/

I do believe we need it based on the expensive cost of tlb shootdown in arm64
even by hardware broadcast.

>
> Thanks.
>
> >> +            return false;> +
> >> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
> >> +    if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
> >> +            return false;
> >> +#endif
> >> +
> >> +    return true;
> >> +}
> >> +
> >
> > [...]
> >
> > .
> >

Thanks
Barry