linux-kernel - Re: [PATCH v3 0/4] mm: arm64: bring up BATCHED_UNMAP_TLB

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Message-ID: <34762b48-3da4-4f2e-64a7-68a44f21d4fd@huawei.com>
Date:   Tue, 6 Sep 2022 16:53:20 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-arm-kernel@...ts.infradead.org>, <x86@...nel.org>,
        <catalin.marinas@....com>, <will@...nel.org>,
        <linux-doc@...r.kernel.org>
CC:     <corbet@....net>, <peterz@...radead.org>, <arnd@...db.de>,
        <linux-kernel@...r.kernel.org>, <darren@...amperecomputing.com>,
        <yangyicong@...ilicon.com>, <huzhanyuan@...o.com>,
        <lipeifeng@...o.com>, <zhangshiming@...o.com>, <guojian@...o.com>,
        <realmz6@...il.com>, <linux-mips@...r.kernel.org>,
        <openrisc@...ts.librecores.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <linux-riscv@...ts.infradead.org>, <linux-s390@...r.kernel.org>,
        Barry Song <21cnbao@...il.com>, <wangkefeng.wang@...wei.com>,
        <xhao@...ux.alibaba.com>, <prime.zeng@...ilicon.com>,
        <anshuman.khandual@....com>
Subject: Re: [PATCH v3 0/4] mm: arm64: bring up BATCHED_UNMAP_TLB_FLUSH

Hi mm and arm64 maintainers,

a gentle ping for this..

Thanks.

On 2022/8/22 16:21, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@...ilicon.com>
> 
> Though ARM64 has the hardware to do tlb shootdown, the hardware
> broadcasting is not free.
> A simplest micro benchmark shows even on snapdragon 888 with only
> 8 cores, the overhead for ptep_clear_flush is huge even for paging
> out one page mapped by only one process:
> 5.36%  a.out    [kernel.kallsyms]  [k] ptep_clear_flush
> 
> While pages are mapped by multiple processes or HW has more CPUs,
> the cost should become even higher due to the bad scalability of
> tlb shootdown.
> 
> The same benchmark can result in 16.99% CPU consumption on ARM64
> server with around 100 cores according to Yicong's test on patch
> 4/4.
> 
> This patchset leverages the existing BATCHED_UNMAP_TLB_FLUSH by
> 1. only send tlbi instructions in the first stage -
> 	arch_tlbbatch_add_mm()
> 2. wait for the completion of tlbi by dsb while doing tlbbatch
> 	sync in arch_tlbbatch_flush()
> My testing on snapdragon shows the overhead of ptep_clear_flush
> is removed by the patchset. The micro benchmark becomes 5% faster
> even for one page mapped by single process on snapdragon 888.
> 
> -v3:
> 1. Declare arch's tlbbatch defer support by arch_tlbbatch_should_defer() instead
>    of ARCH_HAS_MM_CPUMASK, per Barry and Kefeng
> 2. Add Tested-by from Xin Hao
> Link: https://lore.kernel.org/linux-mm/20220711034615.482895-1-21cnbao@gmail.com/
> 
> -v2:
> 1. Collected Yicong's test result on kunpeng920 ARM64 server;
> 2. Removed the redundant vma parameter in arch_tlbbatch_add_mm()
>    according to the comments of Peter Zijlstra and Dave Hansen
> 3. Added ARCH_HAS_MM_CPUMASK rather than checking if mm_cpumask
>    is empty according to the comments of Nadav Amit
> 
> Thanks, Peter, Dave and Nadav for your testing or reviewing
> , and comments.
> 
> -v1:
> https://lore.kernel.org/lkml/20220707125242.425242-1-21cnbao@gmail.com/
> 
> Anshuman Khandual (1):
>   mm/tlbbatch: Introduce arch_tlbbatch_should_defer()
> 
> Barry Song (3):
>   Revert "Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't
>     apply to ARM64"
>   mm: rmap: Extend tlbbatch APIs to fit new platforms
>   arm64: support batched/deferred tlb shootdown during page reclamation
> 
>  Documentation/features/arch-support.txt       |  1 -
>  .../features/vm/TLB/arch-support.txt          |  2 +-
>  arch/arm64/Kconfig                            |  1 +
>  arch/arm64/include/asm/tlbbatch.h             | 12 ++++++++
>  arch/arm64/include/asm/tlbflush.h             | 28 +++++++++++++++++--
>  arch/x86/include/asm/tlbflush.h               | 15 +++++++++-
>  mm/rmap.c                                     | 19 +++++--------
>  7 files changed, 61 insertions(+), 17 deletions(-)
>  create mode 100644 arch/arm64/include/asm/tlbbatch.h
>