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    SubjectRe: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs on stack
    On 03/18/2015 10:22 PM, Andy Lutomirski wrote:
    > On Wed, Mar 18, 2015 at 2:12 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
    >> On 03/18/2015 10:01 PM, Andy Lutomirski wrote:
    >>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@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.

    I did not measure this patch in isolation this time around,
    on the previous iteration of this patch it was a single-cycle speedup.





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