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

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Baoquan He <bhe@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	<x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
	Eric Biederman <ebiederm@xmission.com>,
	"Rob Herring" <robh+dt@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	<devicetree@vger.kernel.org>, Dave Young <dyoung@redhat.com>,
	Vivek Goyal <vgoyal@redhat.com>, <kexec@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, Will Deacon <will@kernel.org>,
	<linux-arm-kernel@lists.infradead.org>,
	Jonathan Corbet <corbet@lwn.net>, <linux-doc@vger.kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Feng Zhou <zhoufeng.zf@bytedance.com>,
	Chen Zhou <dingguo.cz@antgroup.com>,
	"John Donnelly" <John.p.donnelly@oracle.com>,
	Dave Kleikamp <dave.kleikamp@oracle.com>,
	liushixin <liushixin2@huawei.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory
Date: Mon, 27 Jun 2022 19:11:32 +0800	[thread overview]
Message-ID: <5996a53a-70eb-bb30-c8a6-b0c2c8ebb0f9@huawei.com> (raw)
In-Reply-To: <YrmDo7Sx1jNQ4WFd@MiWiFi-R3L-srv>

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

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