On February 7, 2018 9:01:42 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >On Wed, Feb 7, 2018 at 6:59 AM, Kirill A. Shutemov ><kirill.shutemov@xxxxxxxxxxxxxxx> wrote: >> This patchset introduces concept of patchable constants: constant >values >> that can be adjusted at boot-time in response to system configuration >or >> user input (kernel command-line). >> >> Patchable constants can replace variables that never changes at >runtime >> (only at boot-time), but used in very hot path. > >So I actually wanted something very close to this, but I think your >approach is much too simplistic. > >You force all constants into a register, which means that the >resulting code is always going to be very far from non-optimal. > >You also force a big "movabsq" instruction, which really is huge, and >almost never needed. Together with the "use a register", it just makes >for big code. > >What I wanted was something that can take things like a shift by a >variable that is set once, and turn it into a shift by a boot-time >constant. Which means that you literally end up patching the 8-bit >immediate in the shift instruction itself. > >In particular, was looking at the dcache hashing code, and (to quote >an old email of mine), what I wanted was to simplify the run-time >constant part of this: > >│ mov $0x20,%ecx >│ sub 0xaf8bd5(%rip),%ecx # ffffffff81d34600 <d_hash_shift> >│ mov 0x8(%rsi),%r9 >│ add %r14d,%eax >│ imul $0x9e370001,%eax,%eax >│ shr %cl,%eax > >and it was the expression "32-d_hash_shift" that is really a constant, >and that sequence of > >│ mov $0x20,%ecx >│ sub 0xaf8bd5(%rip),%ecx # ffffffff81d34600 <d_hash_shift> >│ shr %cl,%eax > >should be just a single > >│ shr $CONSTANT,%eax > >at runtime. > >Look - much smaller code, and register %rcx isn't used at all. And no >D$ miss on loading that constant (that is a constant depending on >boot-time setup only). > >It's rather more complex, but it actually gives a much bigger win. The >code itself will be much better, and smaller. > >The *infrastructure* for the code gets pretty hairy, though. > >The good news is that the patch already existed to at least _some_ >degree. Peter Anvin did it about 18 months ago. > >It was not really pursued all the way because it *is* a lot of extra >complexity, and I think there was some other hold-up, but he did have >skeleton code for the actual replacement. > >There was a thread on the x86 arch list with the subject line > > Disgusting pseudo-self-modifying code idea: "variable constants" > >but I'm unable to actually find the patch. I know there was at least a >vert early prototype. > >Adding hpa to the cc in the hope that he has some prototype code still >laying around.. > > Linus I am currently working on it much more comprehensive set of patches for extremely this. I am already much further ahead and support most operations. -- Sent from my Android device with K-9 Mail. Please excuse my brevity. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href
