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Fri, 28 Oct 2022 06:11:43 -0700 (PDT) From: Punit Agrawal To: Yicong Yang Cc: Punit Agrawal , Barry Song <21cnbao@gmail.com>, , , , , , , , , , , , , , , , , , , , , , , , Barry Song , Nadav Amit , Mel Gorman , , , , Anshuman Khandual Subject: Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown during page reclamation References: <20220921084302.43631-1-yangyicong@huawei.com> <20220921084302.43631-3-yangyicong@huawei.com> <168eac93-a6ee-0b2e-12bb-4222eff24561@arm.com> <8e391962-4e3a-5a56-64b4-78e8637e3b8c@huawei.com> <87o7tx5oyx.fsf@stealth> Date: Fri, 28 Oct 2022 14:11:41 +0100 In-Reply-To: (Yicong Yang's message of "Fri, 28 Oct 2022 09:20:08 +0800") Message-ID: <87bkpw5bzm.fsf@stealth> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=EFIo+tct; spf=pass (imf26.hostedemail.com: domain of punit.agrawal@bytedance.com designates 209.85.221.47 as permitted sender) smtp.mailfrom=punit.agrawal@bytedance.com; 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spf=pass (imf26.hostedemail.com: domain of punit.agrawal@bytedance.com designates 209.85.221.47 as permitted sender) smtp.mailfrom=punit.agrawal@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Stat-Signature: 53dnd4fe5ckow9ndo1rwji4ahgmpdxfz X-Rspamd-Server: rspam10 X-HE-Tag: 1666962706-568995 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Yicong Yang writes: > On 2022/10/27 22:19, Punit Agrawal wrote: >> >> [ Apologies for chiming in late in the conversation ] >> >> Anshuman Khandual writes: >> >>> 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 ? >> >> 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. [...]