linux-kernel - Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

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Message-ID: <CAGsJ_4z=dZbrAUD9jczT08S3qi_ep-h+EK35UfayVk1S+Cnp2A@mail.gmail.com>
Date:   Wed, 28 Sep 2022 13:23:50 +1300
From:   Barry Song <21cnbao@...il.com>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        yangyicong@...ilicon.com, corbet@....net, peterz@...radead.org,
        arnd@...db.de, linux-kernel@...r.kernel.org,
        darren@...amperecomputing.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, linux-mm@...ck.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, akpm@...ux-foundation.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        wangkefeng.wang@...wei.com, xhao@...ux.alibaba.com,
        prime.zeng@...ilicon.com, Barry Song <v-songbaohua@...o.com>,
        Nadav Amit <namit@...are.com>, Mel Gorman <mgorman@...e.de>,
        catalin.marinas@....com, will@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown
 during page reclamation

On Tue, Sep 27, 2022 at 10:15 PM Yicong Yang <yangyicong@...wei.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.

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.

>
> Thanks.
>
> >> +            return false;> +
> >> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
> >> +    if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
> >> +            return false;
> >> +#endif
> >> +
> >> +    return true;
> >> +}
> >> +
> >
> > [...]
> >
> > .
> >

Thanks
Barry