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Message-ID: <20120119144141.GA547@Krystal>
Date: Thu, 19 Jan 2012 09:41:41 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Frederic Weisbecker <fweisbec@...il.com>,
"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH 2/2] jump labels/x86: Use etiher 5 byte or 2 byte jumps
* Steven Rostedt (rostedt@...dmis.org) wrote:
> From: Steven Rostedt <srostedt@...hat.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@...dmis.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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