On Wed, Mar 18, 2015 at 2:32 PM, Denys Vlasenko <dvlas...@redhat.com> wrote: > On 03/18/2015 10:22 PM, Andy Lutomirski wrote: >> On Wed, Mar 18, 2015 at 2:12 PM, Denys Vlasenko <dvlas...@redhat.com> wrote: >>> On 03/18/2015 10:01 PM, Andy Lutomirski wrote: >>>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlas...@redhat.com> >>>> 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.
Oh, I misunderstood and assumed that the comment there was about that patch in isolation. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/