linux-kernel - Re: [RESEND PATCH v9 2/2] arm64: support batched/deferred tlb shootdown during page reclamation/migration

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Message-ID: <ZJ2x6DlmyA3kVh1n@arm.com>
Date:   Thu, 29 Jun 2023 17:31:36 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
        mark.rutland@....com, ryan.roberts@....com, will@...nel.org,
        anshuman.khandual@....com, linux-doc@...r.kernel.org,
        corbet@....net, peterz@...radead.org, arnd@...db.de,
        punit.agrawal@...edance.com, 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,
        Jonathan.Cameron@...wei.com, Barry Song <v-songbaohua@...o.com>,
        Nadav Amit <namit@...are.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [RESEND PATCH v9 2/2] arm64: support batched/deferred tlb
 shootdown during page reclamation/migration

On Thu, May 18, 2023 at 02:59:34PM +0800, Yicong Yang wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> on x86, batched and deferred tlb shootdown has lead to 90%
> performance increase on tlb shootdown. on arm64, HW can do
> tlb shootdown without software IPI. But sync tlbi is still
> quite expensive.
[...]
>  .../features/vm/TLB/arch-support.txt          |  2 +-
>  arch/arm64/Kconfig                            |  1 +
>  arch/arm64/include/asm/tlbbatch.h             | 12 ++++
>  arch/arm64/include/asm/tlbflush.h             | 33 ++++++++-
>  arch/arm64/mm/flush.c                         | 69 +++++++++++++++++++
>  arch/x86/include/asm/tlbflush.h               |  5 +-
>  include/linux/mm_types_task.h                 |  4 +-
>  mm/rmap.c                                     | 12 ++--

First of all, this patch needs to be split in some preparatory patches
introducing/renaming functions with no functional change for x86. Once
done, you can add the arm64-only changes.

Now, on the implementation, I had some comments on v7 but we didn't get
to a conclusion and the thread eventually died:

https://lore.kernel.org/linux-mm/Y7cToj5mWd1ZbMyQ@arm.com/

I know I said a command line argument is better than Kconfig or some
random number of CPUs heuristics but it would be even better if we don't
bother with any, just make this always on. Barry had some comments
around mprotect() being racy and that's why we have
flush_tlb_batched_pending() but I don't think it's needed (or, for
arm64, it can be a DSB since this patch issues the TLBIs but without the
DVM Sync). So we need to clarify this (see Barry's last email on the
above thread) and before attempting new versions of this patchset. With
flush_tlb_batched_pending() removed (or DSB), I have a suspicion such
implementation would be faster on any SoC irrespective of the number of
CPUs.

-- 
Catalin