linux-kernel - Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory

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Date:   Mon, 27 Jun 2022 17:17:49 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Baoquan He <bhe@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>
CC:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
        "Eric Biederman" <ebiederm@...ssion.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Frank Rowand" <frowand.list@...il.com>,
        <devicetree@...r.kernel.org>, Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, <kexec@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, Will Deacon <will@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Chen Zhou <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>,
        "Dave Kleikamp" <dave.kleikamp@...cle.com>,
        liushixin <liushixin2@...wei.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash
 high memory



On 2022/6/27 10:52, Baoquan He wrote:
> On 06/23/22 at 03:07pm, Catalin Marinas wrote:
>> On Wed, Jun 22, 2022 at 04:35:16PM +0800, Baoquan He wrote:
>>> On 06/21/22 at 07:04pm, Catalin Marinas wrote:
>>>> The problem with splitting is that you can end up with two entries in
>>>> the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
>>>> for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
>>>> abort (but can be worse like loss of coherency).
>>>
>>> Thanks for this explanation. Is this a drawback of arm64 design? X86
>>> code do the same thing w/o issue, is there way to overcome this on
>>> arm64 from hardware or software side?
>>
>> It is a drawback of the arm64 implementations. Having multiple TLB
>> entries for the same VA would need additional logic in hardware to
>> detect, so the microarchitects have pushed back. In ARMv8.4, some
>> balanced was reached with FEAT_BBM so that the only visible side-effect
>> is a potential TLB conflict abort that could be resolved by software.
> 
> I see, thx.
> 
>>
>>> I ever got a arm64 server with huge memory, w or w/o crashkernel setting 
>>> have different bootup time. And the more often TLB miss and flush will
>>> cause performance cost. It is really a pity if we have very powerful
>>> arm64 cpu and system capacity, but bottlenecked by this drawback.
>>
>> Is it only the boot time affected or the runtime performance as well?
> 
> Sorry for late reply. What I observerd is the boot time serious latecy
> with huge memory. Since the timestamp is not available at that time,
> we can't tell the number. I didn't notice the runtime performance.

There's some data here, and I see you're not on the cc list.

https://lore.kernel.org/linux-mm/1656241815-28494-1-git-send-email-guanghuifeng@linux.alibaba.com/T/

> 
> .
> 

-- 
Regards,
  Zhen Lei