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

[Leizhen (ThunderTown)]

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@xxxxxxxxxxxxxxxxx/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