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

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Message-ID: <2AB9EC05-16B4-46F8-B716-53941C1C9A50@vmware.com>
Date:   Thu, 15 Sep 2022 14:31:45 +0000
From:   Nadav Amit <namit@...are.com>
To:     Barry Song <21cnbao@...il.com>
CC:     Anshuman Khandual <anshuman.khandual@....com>,
        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>,
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        "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 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.