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
