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

[Barry Song]

On Thu, Oct 27, 2022 at 11:42 PM Anshuman Khandual
<anshuman.khandual at arm.com> wrote:
>
>
>
> On 9/28/22 05:53, Barry Song wrote:
> > On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong at huawei.com> 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.
>
> No, sounds bit excessive. Kernel command line options should not be added
> for every possible run time switch options.
>
> >
> > 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@linux.alibaba.com/
> >
> > I do believe we need it based on the expensive cost of tlb shootdown in arm64
> > even by hardware broadcast.
>
> Alright, for now could we enable ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH selectively
> with CONFIG_EXPERT and for num_online_cpus()  > 8 ?

Sounds good to me. It is a good start to bring up tlb batched flush in
ARM64. Later on, we
might want to see it in both memory reclamation and migration.

Thanks
Barry