Messages in this thread | | | Date | Thu, 19 Jan 2012 09:41:41 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 2/2] jump labels/x86: Use etiher 5 byte or 2 byte jumps |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > From: Steven Rostedt <srostedt@redhat.com> > > Have the jump labels add a "jmp" in the assembly instead > of a default nop. This will cause the assembler to put in > either a 2 byte or 5 byte jmp depending on where the target > lable is. > > Then at compile time, the update_jump_label code will replace > the jmps with either 2 or 5 byte nops. > > On boot up, the code can be examined to see if the jump label > uses either a 2 or 5 byte nop and replace it. > > By allowing the jump labels to be 2 bytes, it speeds up the > nops, not only 2 byte nops are faster than 5 byte nops, but also > because it saves on cache foot print. > > text data bss dec hex filename > 13403667 3666856 2998272 20068795 13239bb ../nobackup/mxtest/vmlinux-old > 13398536 3666856 2998272 20063664 13225b0 ../nobackup/mxtest/vmlinux-new > > Converting the current v3.2 trace points saved 5,131 bytes. > As more places use jump labels, this will have a bigger savings. > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org> > --- > arch/x86/Kconfig | 1 + > arch/x86/include/asm/jump_label.h | 2 +- > arch/x86/kernel/jump_label.c | 86 ++++++++++++++++++++++++++++++++++--- > 3 files changed, 81 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index efb4294..b5004c1 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -61,6 +61,7 @@ config X86 > select HAVE_ARCH_KMEMCHECK > select HAVE_USER_RETURN_NOTIFIER > select HAVE_ARCH_JUMP_LABEL > + select HAVE_BUILD_TIME_JUMP_LABEL > select HAVE_TEXT_POKE_SMP > select HAVE_GENERIC_HARDIRQS > select HAVE_SPARSE_IRQ > diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h > index a32b18c..872b3e1 100644 > --- a/arch/x86/include/asm/jump_label.h > +++ b/arch/x86/include/asm/jump_label.h > @@ -14,7 +14,7 @@ > static __always_inline bool arch_static_branch(struct jump_label_key *key) > { > asm goto("1:" > - JUMP_LABEL_INITIAL_NOP > + "jmp %l[l_yes]\n"
Is it possible that the compiler choose a jump that is not 2 or 5-byte ? e.g. a jmp rel16 (e9 opcode) on 32-bit x86, or any of the other instruction listed under the JMP-Jump instruction in the Intel insn manual ?
> ".pushsection __jump_table, \"aw\" \n\t" > _ASM_ALIGN "\n\t" > _ASM_PTR "1b, %l[l_yes], %c0 \n\t" > diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c > index ea9d5f2f..d5b84de 100644 > --- a/arch/x86/kernel/jump_label.c > +++ b/arch/x86/kernel/jump_label.c > @@ -16,12 +16,27 @@ > > #ifdef HAVE_JUMP_LABEL > > +static unsigned char nop_short[] = { P6_NOP2 }; > + > +/* These are the nops added at compile time */ > +#ifdef CONFIG_X86_32 > +static unsigned char default_nop[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 }; > +#else > +static unsigned char default_nop[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 }; > +#endif > + > +static int update_nops; > + > union jump_code_union { > char code[JUMP_LABEL_NOP_SIZE]; > struct { > char jump; > int offset; > - } __attribute__((packed)); > + } __packed; > + struct { > + char jump_short; > + char offset_short; > + } __packed; > }; > > static void __jump_label_transform(struct jump_entry *entry, > @@ -29,20 +44,70 @@ static void __jump_label_transform(struct jump_entry *entry, > void *(*poker)(void *, const void *, size_t)) > { > union jump_code_union code; > + unsigned char nop; > + unsigned char op; > + unsigned size; > + void *ip = (void *)entry->code; > + void *ideal = (void *)ideal_nops[NOP_ATOMIC5];
"void *" should possibly be "unsigned char *" here to respect the nop place-holder typing.
> + > + /* Use probe_kernel_read()? */ > + op = *(unsigned char *)ip; > + nop = ideal_nops[NOP_ATOMIC5][0]; > > if (type == JUMP_LABEL_ENABLE) { > - code.jump = 0xe9; > - code.offset = entry->target - > - (entry->code + JUMP_LABEL_NOP_SIZE); > - } else > - memcpy(&code, ideal_nops[NOP_ATOMIC5], JUMP_LABEL_NOP_SIZE); > + if (op == 0xe9 || op == 0xeb) > + /* Already enabled. Warn? */
This could be caused by failure to run the link-time script, or running the transform twice. A warning would indeed be welcome, as this should never happen.
> + return; > + > + if (memcmp(ip, nop_short, 2) == 0) { > + size = 2; > + code.jump_short = 0xeb; > + code.offset = entry->target - > + (entry->code + 2); > + /* Check for overflow ? */ > + } else if (memcmp(ip, ideal, 5) == 0 || > + memcmp(ip, default_nop, 5)) { > + size = JUMP_LABEL_NOP_SIZE; > + code.jump = 0xe9; > + code.offset = entry->target - (entry->code + size); > + } else > + BUG(); > > - (*poker)((void *)entry->code, &code, JUMP_LABEL_NOP_SIZE); > + } else { > + /* Check if already disabled */ > + if (memcmp(ip, nop_short, 2) == 0) > + return; > + > + if (memcmp(ip, ideal, 5) == 0) > + return; > + > + /* This may need to update from default to ideal */ > + if (update_nops && memcmp(ip, default_nop, 5) == 0) { > + /* Set to the ideal nop */ > + size = JUMP_LABEL_NOP_SIZE; > + memcpy(&code, ideal_nops[NOP_ATOMIC5], size);
whiteline.
> + > + } else if (op == 0xe9) { > + /* Replace a 5 byte jmp */ > + size = JUMP_LABEL_NOP_SIZE; > + memcpy(&code, ideal_nops[NOP_ATOMIC5], size); > + } else if (op == 0xeb) { > + /* Replace a 2 byte jmp */ > + size = 2; > + memcpy(&code, nop_short, size); > + } else > + BUG(); > + }
Thanks,
Mathieu
> + > + (*poker)((void *)entry->code, &code, size); > } > > void arch_jump_label_transform(struct jump_entry *entry, > enum jump_label_type type) > { > + /* All nops should be updated to the ideal nop by now */ > + update_nops = 0; > + > get_online_cpus(); > mutex_lock(&text_mutex); > __jump_label_transform(entry, type, text_poke_smp); > @@ -53,6 +118,13 @@ void arch_jump_label_transform(struct jump_entry *entry, > void arch_jump_label_transform_static(struct jump_entry *entry, > enum jump_label_type type) > { > + /* > + * If the default nop does not equal the ideal nop, then > + * update them. > + */ > + if (memcmp(default_nop, ideal_nops[NOP_ATOMIC5], 5) != 0) > + update_nops = 1; > + > __jump_label_transform(entry, type, text_poke_early); > } > > -- > 1.7.8.3 > >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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