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Message-ID: <20140313181033.GH25472@mudshark.cambridge.arm.com>
Date: Thu, 13 Mar 2014 18:10:33 +0000
From: Will Deacon <will.deacon@....com>
To: AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc: "rostedt@...dmis.org" <rostedt@...dmis.org>,
"fweisbec@...il.com" <fweisbec@...il.com>,
"mingo@...hat.com" <mingo@...hat.com>,
Catalin Marinas <Catalin.Marinas@....com>,
"tim.bird@...ymobile.com" <tim.bird@...ymobile.com>,
"gkulkarni@...iumnetworks.com" <gkulkarni@...iumnetworks.com>,
"dsaxena@...aro.org" <dsaxena@...aro.org>,
"arndb@...db.de" <arndb@...db.de>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 5/7] arm64: ftrace: Add dynamic ftrace support
On Thu, Mar 13, 2014 at 10:13:48AM +0000, AKASHI Takahiro wrote:
> This patch allows "dynamic ftrace" if CONFIG_DYNAMIC_FTRACE is enabled.
> Here we can turn on and off tracing dynamically per-function base.
>
> On arm64, this is done by patching single branch instruction to _mcount()
> inserted by gcc -pg option. The branch is replaced to NOP initially at
> kernel start up, and later on, NOP to branch to ftrace_caller() when
> enabled or branch to NOP when disabled.
> Please note that ftrace_caller() is a counterpart of _mcount() in case of
> 'static' ftrace.
>
> More details on architecture specific requirements are described in
> Documentation/trace/ftrace-design.txt.
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> ---
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/ftrace.h | 15 +++++
> arch/arm64/kernel/entry-ftrace.S | 43 +++++++++++++++
> arch/arm64/kernel/ftrace.c | 113 ++++++++++++++++++++++++++++++++++++++
> 4 files changed, 172 insertions(+)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6b3fef6..6954959 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -33,6 +33,7 @@ config ARM64
> select HAVE_DMA_API_DEBUG
> select HAVE_DMA_ATTRS
> select HAVE_DMA_CONTIGUOUS
> + select HAVE_DYNAMIC_FTRACE
> select HAVE_EFFICIENT_UNALIGNED_ACCESS
> select HAVE_FTRACE_MCOUNT_RECORD
> select HAVE_FUNCTION_TRACER
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 58ea595..ed5c448 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -18,6 +18,21 @@
>
> #ifndef __ASSEMBLY__
> extern void _mcount(unsigned long);
> +
> +struct dyn_arch_ftrace {
> + /* No extra data needed for arm64 */
> +};
> +
> +extern unsigned long ftrace_graph_call;
> +
> +static inline unsigned long ftrace_call_adjust(unsigned long addr)
> +{
> + /*
> + * addr is the address of the mcount call instruction.
> + * recordmcount does the necessary offset calculation.
> + */
> + return addr;
> +}
You could just as easily implement this as a dummy macro, but I guess it
doesn't matter either way.
> #endif /* __ASSEMBLY__ */
>
> #endif /* __ASM_FTRACE_H */
> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
> index 0ac31c8..c0fbe10 100644
> --- a/arch/arm64/kernel/entry-ftrace.S
> +++ b/arch/arm64/kernel/entry-ftrace.S
> @@ -86,6 +86,7 @@
> add \reg, \reg, #8
> .endm
>
> +#ifndef CONFIG_DYNAMIC_FTRACE
> /*
> * void _mcount(unsigned long return_address)
> * @return_address: return address to instrumented function
> @@ -134,6 +135,48 @@ skip_ftrace_call:
> #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> ENDPROC(_mcount)
>
> +#else /* CONFIG_DYNAMIC_FTRACE */
> +/*
> + * _mcount() is used to build the kernel with -pg option, but all the branch
> + * instructions to _mcount() are replaced to NOP initially at kernel start up,
> + * and later on, NOP to branch to ftrace_caller() when enabled or branch to
> + * NOP when disabled per-function base.
> + */
> +ENTRY(_mcount)
> + ret
> +ENDPROC(_mcount)
Judging by your comment then, this should never be called. Is that right? If
so, we could add a BUG-equivalent so we know if we missed an mcount during
patching.
> +/*
> + * void ftrace_caller(unsigned long return_address)
> + * @return_address: return address to instrumented function
> + *
> + * This function is a counterpart of _mcount() in 'static' ftrace, and
> + * makes calls to:
> + * - tracer function to probe instrumented function's entry,
> + * - ftrace_graph_caller to set up an exit hook
> + */
> +ENTRY(ftrace_caller)
> + mcount_enter
> +
> + mcount_get_pc0 x0 // function's pc
> + mcount_get_lr x1 // function's lr
> +
> + .global ftrace_call
> +ftrace_call: // tracer(pc, lr);
> + nop // This will be replaced with "bl xxx"
> + // where xxx can be any kind of tracer.
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> + .global ftrace_graph_call
> +ftrace_graph_call: // ftrace_graph_caller();
> + nop // If enabled, this will be replaced
> + // "b ftrace_graph_caller"
> +#endif
> +
> + mcount_exit
> +ENDPROC(ftrace_caller)
> +#endif /* CONFIG_DYNAMIC_FTRACE */
> +
> ENTRY(ftrace_stub)
> ret
> ENDPROC(ftrace_stub)
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index a559ab8..8c26476 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -17,6 +17,89 @@
> #include <asm/ftrace.h>
> #include <asm/insn.h>
>
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +/*
> + * Replace a single instruction, which may be a branch or NOP.
> + * If @validate == true, a replaced instruction is checked against 'old'.
> + */
> +static int ftrace_modify_code(unsigned long pc, unsigned int old,
> + unsigned int new, bool validate)
> +{
> + unsigned int replaced;
u32 is a bit clearer for instructions.
> + /*
> + * Note:
> + * Due to modules and __init, code can disappear and change,
> + * we need to protect against faulting as well as code changing.
> + * We do this by aarch64_insn_*() which use the probe_kernel_*().
> + *
> + * No lock is held here because all the modifications are run
> + * through stop_machine().
> + */
> + if (validate) {
> + if (aarch64_insn_read((void *)pc, &replaced))
> + return -EFAULT;
> +
> + if (replaced != old)
> + return -EINVAL;
> + }
> + if (aarch64_insn_patch_text_nosync((void *)pc, new))
> + return -EPERM;
I think you're better off propagating the errors here, rather than
overriding them with EFAULT/EINVAL/EPERM.
> +
> + return 0;
> +}
> +
> +/*
> + * Replace tracer function in ftrace_caller()
> + */
> +int ftrace_update_ftrace_func(ftrace_func_t func)
> +{
> + unsigned long pc;
> + unsigned int new;
> +
> + pc = (unsigned long)&ftrace_call;
> + new = aarch64_insn_gen_branch_imm(pc, (unsigned long)func, true);
> +
> + return ftrace_modify_code(pc, 0, new, false);
> +}
> +
> +/*
> + * Turn on the call to ftrace_caller() in instrumented function
> + */
> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> +{
> + unsigned long pc = rec->ip;
> + unsigned int old, new;
> +
> + old = aarch64_insn_gen_nop();
> + new = aarch64_insn_gen_branch_imm(pc, addr, true);
> +
> + return ftrace_modify_code(pc, old, new, true);
> +}
> +
> +/*
> + * Turn off the call to ftrace_caller() in instrumented function
> + */
> +int ftrace_make_nop(struct module *mod,
> + struct dyn_ftrace *rec, unsigned long addr)
> +{
> + unsigned long pc = rec->ip;
> + unsigned int old, new;
> +
> + old = aarch64_insn_gen_branch_imm(pc, addr, true);
> + new = aarch64_insn_gen_nop();
> +
> + return ftrace_modify_code(pc, old, new, true);
> +}
> +
> +int __init ftrace_dyn_arch_init(void *data)
> +{
> + *(unsigned long *)data = 0;
> +
> + return 0;
> +}
> +#endif /* CONFIG_DYNAMIC_FTRACE */
> +
> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> /*
> * function_graph tracer expects ftrace_return_to_handler() to be called
> @@ -61,4 +144,34 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
> return;
> }
> }
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +/*
> + * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
> + * depending on @enable.
> + */
> +static int ftrace_modify_graph_caller(bool enable)
> +{
> + unsigned long pc = (unsigned long)&ftrace_graph_call;
> + unsigned int branch, nop, old, new;
> +
> + branch = aarch64_insn_gen_branch_imm(pc,
> + (unsigned long)ftrace_graph_caller, false);
> + nop = aarch64_insn_gen_nop();
> + old = enable ? nop : branch;
> + new = enable ? branch : nop;
> +
> + return ftrace_modify_code(pc, old, new, true);
You could rewrite this as:
if (enable)
return ftrace_modify_code(pc, nop, branch, true);
else
return ftrace_modify_code(pc, branch, nop, true);
which I find easier to read.
Will
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