Le 30/11/2023 à 22:30, Peter Xu a écrit : > On Fri, Nov 24, 2023 at 11:07:51AM -0500, Peter Xu wrote: >> On Fri, Nov 24, 2023 at 09:06:01AM +0000, Ryan Roberts wrote: >>> I don't have any micro-benchmarks for GUP though, if that's your question. Is >>> there an easy-to-use test I can run to get some numbers? I'd be happy to try it out. >> >> Thanks Ryan. Then nothing is needed to be tested if gup is not yet touched >> from your side, afaict. I'll see whether I can provide some rough numbers >> instead in the next post (I'll probably only be able to test it in a VM, >> though, but hopefully that should still reflect mostly the truth). > > An update: I finished a round of 64K cont_pte test, in the slow gup micro > benchmark I see ~15% perf degrade with this patchset applied on a VM on top > of Apple M1. > > Frankly that's even less than I expected, considering not only how slow gup > THP used to be, but also on the fact that that's a tight loop over slow > gup, which in normal cases shouldn't happen: "present" ptes normally goes > to fast-gup, while !present goes into a fault following it. I assume > that's why nobody cared slow gup for THP before. I think adding cont_pte > support shouldn't be very hard, but that will include making cont_pte idea > global just for arm64 and riscv Svnapot. Is there any documentation on what cont_pte is ? I have always wondered if it could also fit powerpc 8xx need ? On powerpc, for 16k pages, we have to define 4 consecutive PTEs. All 4 PTE are flagged with the SPS bit telling it's a 16k pages, but for TLB misses the HW needs one entrie for each 4k fragment. There is also a similar approach for 512k pages, we have 128 contiguous identical PTEs for them. And whatever PAGE_SIZE is (either 4k or 16k), the HW needs one 'unsigned long' pte for each 4k fragment. So at the time being when we define PAGE_SIZE as 16k, we need a special pte_t which is a table of 4x unsigned long. Wondering if the cont_pte concept is similar and whether it could help. Thanks Christophe
