On 03/18/2015 10:22 PM, Andy Lutomirski wrote: > On Wed, Mar 18, 2015 at 2:12 PM, Denys Vlasenko <[email protected]> wrote: >> On 03/18/2015 10:01 PM, Andy Lutomirski wrote: >>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <[email protected]> >>> wrote: >>>> We lose a number of large insns there: >>>> >>>> text data bss dec hex filename >>>> 9863 0 0 9863 2687 entry_64_before.o >>>> 9671 0 0 9671 25c7 entry_64.o >>>> >>>> What's more important, we convert two "MOVQ $imm,off(%rsp)" to "PUSH $imm" >>>> (the ones which fill pt_regs->cs,ss). >>>> >>>> Before this patch, placing them on fast path was slowing it down by two >>>> cycles: >>>> this form of MOV is very large, 12 bytes, and this probably reduces decode >>>> bandwidth >>>> to one insn per cycle when it meets them. >>>> Therefore they were living in FIXUP_TOP_OF_STACK instead (away from hot >>>> path). >>> >>> Does that mean that this has zero performance impact, or is it >>> actually a speedup? >> >> >> No, it's not a speedup because those big bad instructions weren't >> on hot path to begin with. >> >> We want them to be there. >> >> Inserting them in a form of MOVs into hot path (say, in order >> to eliminate FIXUP_TOP_OF_STACK) *would be* a slowdown. >> >> But we switch to PUSH method, and then inserting them _as PUSHes_ >> seems to be a wash. >> > > Sorry, what I meant was: what was the performance impact of this patch > on fast-path syscalls?
I measured the next patch (which added one additional push) and it was a wash compared to timings before both patches. See comment there. I did not measure this patch in isolation this time around, on the previous iteration of this patch it was a single-cycle speedup. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
