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

[Barry Song]

On Fri, Oct 28, 2022 at 3:19 AM Punit Agrawal
<punit.agrawal@xxxxxxxxxxxxx> wrote:
>
>
> [ Apologies for chiming in late in the conversation ]
>
> Anshuman Khandual <anshuman.khandual@xxxxxxx> writes:
>
> > On 9/28/22 05:53, Barry Song wrote:
> >> 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.
> >
> > 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@xxxxxxxxxxxxxxxxx/
> >>
> >> 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 ?
>
> When running the test program in the commit in a VM, I saw benefits from
> the patches at all sizes from 2, 4, 8, 32 vcpus. On the test machine,
> ptep_clear_flush() went from ~1% in the unpatched version to not showing
> up.
>
> Yicong mentioned that he didn't see any benefit for <= 4 CPUs but is
> there any overhead? I am wondering what are the downsides of enabling
> the config by default.

As we are deferring tlb flush, but sometimes while we are modifying the vma
which are deferred, we need to do a sync by flush_tlb_batched_pending() in
mprotect() , madvise() to make sure they can see the flushed result. if nobody
is doing mprotect(), madvise() etc in the deferred period, the overhead is zero.

>
> Thanks,
> Punit

Thanks
Barry