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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 10/28/22 03:37, Barry Song wrote: > On Thu, Oct 27, 2022 at 11:42 PM Anshuman Khandual > wrote: >> >> >> >> On 9/28/22 05:53, Barry Song wrote: >>> On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang 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. Right, that is the idea, CONFIG_EXPERT gives an way to test this out for some time on various platforms, and later it can be dropped off. Regarding num_online_cpus() = '8' as the threshold which would potentially give benefit of batched TLB should be defined as a macro e.g NR_CPUS_FOR_BATCHED_TLB or internal (non user selectable) config , with a proper in-code comment, explaining the rationale. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel