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? --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/
