linux-kernel - Re: [PATCH v3 4/4] arm64: support batched/deferred tlb shootdown during page reclamation

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Date:   Mon, 19 Sep 2022 08:16:32 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Nadav Amit <namit@...are.com>, Barry Song <21cnbao@...il.com>
Cc:     Yicong Yang <yangyicong@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "corbet@....net" <corbet@....net>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "darren@...amperecomputing.com" <darren@...amperecomputing.com>,
        "yangyicong@...ilicon.com" <yangyicong@...ilicon.com>,
        "huzhanyuan@...o.com" <huzhanyuan@...o.com>,
        "lipeifeng@...o.com" <lipeifeng@...o.com>,
        "zhangshiming@...o.com" <zhangshiming@...o.com>,
        "guojian@...o.com" <guojian@...o.com>,
        "realmz6@...il.com" <realmz6@...il.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "wangkefeng.wang@...wei.com" <wangkefeng.wang@...wei.com>,
        "xhao@...ux.alibaba.com" <xhao@...ux.alibaba.com>,
        "prime.zeng@...ilicon.com" <prime.zeng@...ilicon.com>,
        Barry Song <v-songbaohua@...o.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v3 4/4] arm64: support batched/deferred tlb shootdown
 during page reclamation



On 9/15/22 20:01, Nadav Amit wrote:
> 
> 
>> On Sep 14, 2022, at 11:42 PM, Barry Song <21cnbao@...il.com> wrote:
>>
>>>
>>> The very idea behind TLB deferral is the opportunity it (might) provide
>>> to accumulate address ranges and cpu masks so that individual TLB flush
>>> can be replaced with a more cost effective range based TLB flush. Hence
>>> I guess unless address range or cpumask based cost effective TLB flush
>>> is available, deferral does not improve the unmap performance as much.
>>
>>
>> After sending tlbi, if we wait for the completion of tlbi, we have to get Ack
>> from all cpus in the system, tlbi is not scalable. The point here is that we
>> avoid waiting for each individual TLBi. Alternatively, they are batched. If
>> you read the benchmark in the commit log, you can find the great decline
>> in the cost to swap out a page.
> 
> Just a minor correction: arch_tlbbatch_flush() does not collect ranges.
> On x86 it only accumulate CPU mask.

Thanks Nadav for the clarification.