Date: Mon, 11 Dec 2017 10:15:58 -0800 (PST)
Subject: Re: [patches] [PATCH v2] riscv/ftrace: Add basic support
In-Reply-To: <20171207023110.2622-1-alankao@andestech.com>
CC: albert@sifive.com, rostedt@goodmis.org, mingo@redhat.com,
        patches@groups.riscv.org, linux-kernel@vger.kernel.org,
        greentime@andestech.com, alankao@andestech.com, pombredanne@nexb.com
From: Palmer Dabbelt <palmer@sifive.com>
To: nonerkao@gmail.com, Jim Wilson <jimw@sifive.com>
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On Wed, 06 Dec 2017 18:31:10 PST (-0800), nonerkao@gmail.com wrote:
> This patch contains basic ftrace support for RV64I platform.
> Specifically, function tracer (HAVE_FUNCTION_TRACER), function graph
> tracer (HAVE_FUNCTION_GRAPH_TRACER), and a frame pointer test
> (HAVE_FUNCTION_GRAPH_FP_TEST) are implemented following the
> instructions in Documentation/trace/ftrace-design.txt.
>
> Note that the functions in both ftrace.c and setup.c should not be
> hooked with the compiler's -pg option: to prevent infinite self-
> referencing for the former, and to ignore early setup stuff for the
> latter.
>
> Signed-off-by: Alan Kao <alankao@andestech.com>
> ---
> Changes in v2:
>   - Add SPDX license identifier
>   - Remove unnecessary LOCKDEP_SUPPORT option
>   - Remove unnecessary #ifdef in the newly added kernel/ftrace.c
>
>  arch/riscv/Kconfig              |   5 ++
>  arch/riscv/include/asm/Kbuild   |   1 -
>  arch/riscv/include/asm/ftrace.h |  10 ++++
>  arch/riscv/kernel/Makefile      |   7 +++
>  arch/riscv/kernel/ftrace.c      |  41 +++++++++++++
>  arch/riscv/kernel/mcount.S      | 126 ++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 189 insertions(+), 1 deletion(-)
>  create mode 100644 arch/riscv/include/asm/ftrace.h
>  create mode 100644 arch/riscv/kernel/ftrace.c
>  create mode 100644 arch/riscv/kernel/mcount.S
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 2e15e85c8f7e..40a67fc12328 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -60,6 +60,9 @@ config PAGE_OFFSET
>  config STACKTRACE_SUPPORT
>  	def_bool y
>
> +config TRACE_IRQFLAGS_SUPPORT
> +	def_bool y
> +
>  config RWSEM_GENERIC_SPINLOCK
>  	def_bool y
>
> @@ -112,6 +115,8 @@ config ARCH_RV64I
>  	bool "RV64I"
>  	select CPU_SUPPORTS_64BIT_KERNEL
>  	select 64BIT
> +	select HAVE_FUNCTION_TRACER
> +	select HAVE_FUNCTION_GRAPH_TRACER
>
>  endchoice
>
> diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
> index 18158be62a2b..680301bfbc4b 100644
> --- a/arch/riscv/include/asm/Kbuild
> +++ b/arch/riscv/include/asm/Kbuild
> @@ -12,7 +12,6 @@ generic-y += errno.h
>  generic-y += exec.h
>  generic-y += fb.h
>  generic-y += fcntl.h
> -generic-y += ftrace.h
>  generic-y += futex.h
>  generic-y += hardirq.h
>  generic-y += hash.h
> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> new file mode 100644
> index 000000000000..66d4175eb13e
> --- /dev/null
> +++ b/arch/riscv/include/asm/ftrace.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2017 Andes Technology Corporation */
> +
> +/*
> + * The graph frame test is not possible if CONFIG_FRAME_POINTER is not enabled.
> + * Check arch/riscv/kernel/mcount.S for detail.
> + */
> +#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && defined(CONFIG_FRAME_POINTER)
> +#define HAVE_FUNCTION_GRAPH_FP_TEST
> +#endif
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index ab8baf7bd142..15941f3b8363 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -2,6 +2,11 @@
>  # Makefile for the RISC-V Linux kernel
>  #
>
> +#ifdef CONFIG_FTRACE
> +CFLAGS_REMOVE_ftrace.o = -pg
> +CFLAGS_REMOVE_setup.o = -pg
> +#endif
> +

You want ifeq, not #ifdef, in Makefiles.

Also: I thought we were only unable to trace setup_vm()?  Either way, I don't 
think it's a big deal to avoid tracing anything in setup.c: the stuff in here 
is called once pet hart before userspace is set up, so I doubt anyone is going 
to want to trace it anyway.

>  extra-y += head.o
>  extra-y += vmlinux.lds
>
> @@ -29,5 +34,7 @@ CFLAGS_setup.o := -mcmodel=medany
>  obj-$(CONFIG_SMP)		+= smpboot.o
>  obj-$(CONFIG_SMP)		+= smp.o
>  obj-$(CONFIG_MODULES)		+= module.o
> +obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o
> +obj-$(CONFIG_FUNCTION_GRAPH_TRACER)	+= ftrace.o
>
>  clean:
> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> new file mode 100644
> index 000000000000..d0de68d144cb
> --- /dev/null
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -0,0 +1,41 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2013 Linaro Limited
> + * Author: AKASHI Takahiro <takahiro.akashi@linaro.org>
> + * Copyright (C) 2017 Andes Technology Corporation
> + */
> +
> +#include <linux/ftrace.h>
> +
> +/*
> + * Most of this file is copied from arm64.
> + */
> +void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
> +			   unsigned long frame_pointer)
> +{
> +	unsigned long return_hooker = (unsigned long)&return_to_handler;
> +	unsigned long old;
> +	struct ftrace_graph_ent trace;
> +	int err;
> +
> +	if (unlikely(atomic_read(&current->tracing_graph_pause)))
> +		return;
> +
> +	/*
> +	 * We don't suffer access faults, so no extra fault-recovery assembly
> +	 * is needed here.
> +	 */
> +	old = *parent;
> +
> +	trace.func = self_addr;
> +	trace.depth = current->curr_ret_stack + 1;
> +
> +	if (!ftrace_graph_entry(&trace))
> +		return;
> +
> +	err = ftrace_push_return_trace(old, self_addr, &trace.depth,
> +				       frame_pointer, NULL);
> +	if (err == -EBUSY)
> +		return;
> +	*parent = return_hooker;
> +}

This looks almost exactly like the arm64 version, except they're setting 
"*parent = old" a few more times.  I can't actually figure out why: they also 
set "old = *parent", and they never seem to modify it.  Can you explain the 
discrepancy?

> diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
> new file mode 100644
> index 000000000000..32f2432202e8
> --- /dev/null
> +++ b/arch/riscv/kernel/mcount.S
> @@ -0,0 +1,126 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2017 Andes Technology Corporation */
> +
> +#include <linux/init.h>
> +#include <linux/linkage.h>
> +#include <asm/asm.h>
> +#include <asm/csr.h>
> +#include <asm/unistd.h>
> +#include <asm/thread_info.h>
> +#include <asm/asm-offsets.h>
> +#include <asm-generic/export.h>
> +#include <asm/ftrace.h>
> +
> +	.text
> +
> +	.macro SAVE_ABI_STATE
> +	addi	sp, sp, -16
> +	sd	s0, 0(sp)
> +	sd	ra, 8(sp)
> +	addi	s0, sp, 16
> +	.endm
> +
> +	/*
> +	 * The call to ftrace_return_to_handler would overwrite the return
> +	 * register if a0 was not saved.
> +	 */
> +	.macro SAVE_RET_ABI_STATE
> +	addi	sp, sp, -32
> +	sd	s0, 16(sp)
> +	sd	ra, 24(sp)

Is there any reason these happen out of order?

> +	sd	a0, 8(sp)
> +	addi	s0, sp, 32
> +	.endm
> +
> +	.macro STORE_ABI_STATE
> +	ld	ra, 8(sp)
> +	ld	s0, 0(sp)
> +	addi	sp, sp, 16
> +	.endm
> +
> +	.macro STORE_RET_ABI_STATE
> +	ld	ra, 24(sp)
> +	ld	s0, 16(sp)
> +	ld	a0, 8(sp)
> +	addi	sp, sp, 32
> +	.endm
> +
> +ENTRY(ftrace_stub)
> +	ret
> +ENDPROC(ftrace_stub)
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +ENTRY(return_to_handler)
> +/*
> + * On implementing the frame point test, the ideal way is to compare the
> + * s0 (frame pointer, if enabled) on entry and the sp (stack pointer) on return.
> + * However, the psABI of variable-length-argument functions does not allow this.
> + *
> + * So alternatively we check the *old* frame pointer position, that is, the
> + * value stored in -16(s0) on entry, and the s0 on return.
> + */
> +#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> +	mv	t6, s0
> +#endif
> +	SAVE_RET_ABI_STATE
> +#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> +	mv	a0, t6
> +#endif
> +	la	t0, ftrace_return_to_handler
> +	jalr	t0
> +	mv	a1, a0
> +	STORE_RET_ABI_STATE
> +	jr	a1
> +ENDPROC(return_to_handler)
> +EXPORT_SYMBOL(return_to_handler)
> +#endif
> +
> +ENTRY(_mcount)
> +	la	t4, ftrace_stub
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +	la	t0, ftrace_graph_return
> +	ld	t1, 0(t0)
> +	bne	t1, t4, do_ftrace_graph_caller
> +
> +	la	t3, ftrace_graph_entry
> +	ld	t2, 0(t3)
> +	la	t6, ftrace_graph_entry_stub
> +	bne	t2, t6, do_ftrace_graph_caller
> +#endif
> +	la	t3, ftrace_trace_function
> +	ld	t5, 0(t3)
> +	bne	t5, t4, do_trace
> +	ret

Since this is a performance-critical function, it'd be good to have it 
optimized.  I notice two things:

* There's no early out.
* You can save an instruction when addressing by using somethingl like "ld t1, 
  ftrace_graph_return" instead of "la t0, ftrace_graph_return; ld t1 0(t0)".

It's not a big deal, though -- we can fix these later.  The more interesting 
thing here is that this code means our `-pg` stuff is now part of the GCC ABI, 
which is something I'd never though of before.  I've added Jim, our GCC guy.

Jim: do you mind checking to make sure the GCC profiling support is sane?  
Specifically, I'm thinking:

* Are there any profiling features we don't support that would require an ABI 
 break?
* Is there a way to add future ISA extensions without breaking the ABI?
* Should we document this as part of the ELF psABI specification?

Even though this isn't user-visible as far an Linux is concerned, it'd be a bit 
of a pain to have to break this ABI because we did something brain-dead.  Since 
there's a bit of time before 7.3.0, I think it'd be OK to consider breaking the 
profiling ABI if there's a good reason.

> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +/*
> + * A pseudo representation for the function graph tracer:
> + * prepare_to_return(&ra_to_caller_of_caller, ra_to_caller)
> + */
> +do_ftrace_graph_caller:
> +	addi	a0, s0, -8
> +	mv	a1, ra
> +#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> +	ld	a2, -16(s0)
> +#endif
> +	SAVE_ABI_STATE
> +	la	t0, prepare_ftrace_return
> +	jalr	t0
> +	STORE_ABI_STATE
> +	ret
> +#endif
> +
> +/*
> + * A pseudo representation for the function tracer:
> + * (*ftrace_trace_function)(ra_to_caller, ra_to_caller_of_caller)
> + */
> +do_trace:
> +	ld	a1, -8(s0)
> +	mv	a0, ra
> +
> +	SAVE_ABI_STATE
> +	jalr	t5
> +	STORE_ABI_STATE
> +	ret
> +ENDPROC(_mcount)
> +EXPORT_SYMBOL(_mcount)

Thanks!

I don't know much about ftrace, but I'm OK taking this into our 
"for-linux-next" tree so this gets more visibility.  As we get userspace a bit 
saner then I'll give it another look.

I've put this here for now:

  https://github.com/riscv/riscv-linux/pull/117