Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 18 Mar 2015 14:22:07 -0700 | Subject | Re: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs on stack |
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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?
--Andy
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