Hi, On Tue, Dec 22, 2020 at 01:24:53PM +1000, Nicholas Piggin wrote: > Excerpts from Hugh Dickins's message of December 22, 2020 4:21 am: > > Hi Nick, > > > > On Sun, 20 Dec 2020, Nicholas Piggin wrote: > > > >> Similarly to the previous patch, this tries to optimise dirty/accessed > >> bits in ptes to avoid access costs of hardware setting them. > >> > >> This tidies up a few last cases where dirty/accessed faults can be seen, > >> and subsumes the pte_sw_mkyoung helper -- it's not just architectures > >> with explicit software dirty/accessed bits that take expensive faults to > >> modify ptes. > >> > >> The vast majority of the remaining dirty/accessed faults on kbuild > >> workloads after this patch are from NUMA migration, due to > >> remove_migration_pte inserting old/clean ptes. > > > > Are you sure about this patch? It looks wrong to me: because isn't > > _PAGE_ACCESSED (young) already included in the vm_page_prot __S001 etc? > > > > I haven't checked all instances below, but in general, that's why the > > existing code tends not to bother to mkyoung, but does sometimes mkold > > (admittedly confusing). > > There might have been one or two cases where it didn't come directly > from vm_page_prot, but it was a few rebases and updates ago. I did see > one or two places where powerpc was taking faults. Good point though I > can test again and see, and I might split the patch. > > > > > A quick check on x86 and powerpc looks like they do have _PAGE_ACCESSED > > in there; and IIRC that's true, or ought to be true, of all architectures. > > > > Maybe not mips, which I see you have singled out: I remember going round > > this loop a few months ago with mips, where strange changes were being > > proposed to compensate for not having that bit in their vm_page_prot. > > I didn't follow through to see how that ended up, but I did suggest > > mips needed to follow the same convention as the other architectures. > > Yeah the main thing is to try get all architectures doing the same thing > and get rid of that sw_mkyoung too. Given the (Intel) x86 result of the > heavy micro-fault, I don't think anybody is special and we should > require them to follow the same behaviour unless it's proven that one > needs something different. > > If we can get all arch to set accessed in vm_page_prot and get rid of > most of these mkyoung()s then all the better. And it actually looks like > MIPS may be changing direction: > > https://lore.kernel.org/linux-arch/20200919074731.22372-1-huangpei@xxxxxxxxxxx/ > > What's the chances of that going upstream for the next merge window? It > seems like the right thing to do. Any comment on V4: https://lore.kernel.org/linux-arch/20201019081257.32127-1-huangpei@xxxxxxxxxxx/ > > Thanks, > Nick Thanks, Huang Pei
