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Date:   Fri, 28 Oct 2022 07:26:44 +0530
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Subject: Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown
 during page reclamation
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To:     Barry Song <21cnbao@gmail.com>
Cc:     Yicong Yang <yangyicong@huawei.com>, yangyicong@hisilicon.com,
        corbet@lwn.net, peterz@infradead.org, arnd@arndb.de,
        linux-kernel@vger.kernel.org, darren@os.amperecomputing.com,
        huzhanyuan@oppo.com, lipeifeng@oppo.com, zhangshiming@oppo.com,
        guojian@oppo.com, realmz6@gmail.com, linux-mips@vger.kernel.org,
        openrisc@lists.librecores.org, linux-mm@kvack.org, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, akpm@linux-foundation.org,
        linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org,
        wangkefeng.wang@huawei.com, xhao@linux.alibaba.com,
        prime.zeng@hisilicon.com, Barry Song <v-songbaohua@oppo.com>,
        Nadav Amit <namit@vmware.com>, Mel Gorman <mgorman@suse.de>,
        catalin.marinas@arm.com, will@kernel.org, linux-doc@vger.kernel.org
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>
 <CAGsJ_4z=dZbrAUD9jczT08S3qi_ep-h+EK35UfayVk1S+Cnp2A@mail.gmail.com>
 <ecd161db-b290-7997-a81e-a0a00bd1c599@arm.com>
 <CAGsJ_4x0KhEjm5a9jhtS+YK1AT49u3sHnp2rHZVSuTGZp4nKzA@mail.gmail.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <CAGsJ_4x0KhEjm5a9jhtS+YK1AT49u3sHnp2rHZVSuTGZp4nKzA@mail.gmail.com>
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On 10/28/22 03:37, Barry Song wrote:
> On Thu, Oct 27, 2022 at 11:42 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>>
>>
>> On 9/28/22 05:53, Barry Song wrote:
>>> On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong@huawei.com> 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.