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

[Baoquan He]

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,
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.