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

[Barry Song]

On Sat, Oct 29, 2022 at 2:11 AM Punit Agrawal
<punit.agrawal@xxxxxxxxxxxxx> wrote:
>
> Yicong Yang <yangyicong@xxxxxxxxxx> writes:
>
> > On 2022/10/27 22:19, Punit Agrawal 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.
> >>
> >
> > Maybe you're booting VM on a server with more than 32 cores and Barry tested
> > on his 4 CPUs embedded platform. I guess a 4 CPU VM is not fully equivalent to
> > a 4 CPU real machine as the tbli and dsb in the VM may influence the host
> > as well.
>
> Yeah, I also wondered about this.
>
> I was able to test on a 6-core RK3399 based system - there the
> ptep_clear_flush() was only 0.10% of the overall execution time. The
> hardware seems to do a pretty good job of keeping the TLB flushing
> overhead low.

RK3399 has Dual-core ARM Cortex-A72 MPCore processor and
Quad-core ARM Cortex-A53 MPCore processor. you are probably
going to see different overhead of ptep_clear_flush() when you
bind the micro-benchmark on different cores.

>
> [...]
>

Thanks
Barry