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 19:11:32 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Baoquan He <bhe@...hat.com>
CC:     Catalin Marinas <catalin.marinas@....com>,
        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 18:17, Baoquan He wrote:
> On 06/27/22 at 05:17pm, Leizhen (ThunderTown) wrote:
>>
>>
>> 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/
> 
> Thanks, Zhen Lei. I also saw the patch. That seems to be a good way,

Yes.

> since there's only one process running at that time. Not sure if there's
> still risk of multiple TLB entries for the same VA existing.
> 
> .
> 

-- 
Regards,
  Zhen Lei