On Tue, Jul 09, 2019 at 11:02:40AM -0700, Alexei Starovoitov wrote: > On Tue, Jul 9, 2019 at 10:48 AM Josh Poimboeuf <jpoim...@redhat.com> 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 <a...@kerrnel.org> > > 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