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 ?