On Tue, Jul 09, 2019 at 11:02:40AM -0700, Alexei Starovoitov wrote:
> On Tue, Jul 9, 2019 at 10:48 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > On Mon, Jul 08, 2019 at 04:16:25PM -0700, Alexei Starovoitov wrote:
> > > total time is hard to compare.
> > > Could you compare few tests?
> > > like two that are called "tcpdump *"
> > >
> > > I think small regression is ok.
> > > Folks that care about performance should be using JIT.
> >
> > I did each test 20 times and computed the averages:
> >
> > "tcpdump port 22":
> > default: 0.00743175s
> > -fno-gcse: 0.00709920s (~4.5% speedup)
> >
> > "tcpdump complex":
> > default: 0.00876715s
> > -fno-gcse: 0.00854895s (~2.5% speedup)
> >
> > So there does seem to be a small performance gain by disabling this
> > optimization.
>
> great. thanks for checking.
>
> > We could change it for the whole file, by adjusting CFLAGS_core.o in the
> > BPF makefile, or we could change it for the function only with something
> > like the below patch.
> >
> > Thoughts?
> >
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index e8579412ad21..d7ee4c6bad48 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -170,3 +170,5 @@
> > #else
> > #define __diag_GCC_8(s)
> > #endif
> > +
> > +#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > index 095d55c3834d..599c27b56c29 100644
> > --- a/include/linux/compiler_types.h
> > +++ b/include/linux/compiler_types.h
> > @@ -189,6 +189,10 @@ struct ftrace_likely_data {
> > #define asm_volatile_goto(x...) asm goto(x)
> > #endif
> >
> > +#ifndef __no_fgcse
> > +# define __no_fgcse
> > +#endif
> > +
> > /* Are two types/vars the same type (ignoring qualifiers)? */
> > #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
> >
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 7e98f36a14e2..8191a7db2777 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1295,7 +1295,7 @@ bool bpf_opcode_in_insntable(u8 code)
> > *
> > * Decode and execute eBPF instructions.
> > */
> > -static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > +static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
>
> I prefer per-function flag.
> If you want to route it via tip:
> Acked-by: Alexei Starovoitov <ast@xxxxxxxxxxx>
>
> or Daniel can take it into bpf tree while I'm traveling.
Thanks! I''ll probably send it through the tip tree, along with an
objtool fix for the other optimization.
--
Josh
