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.
