Date:   Wed, 4 Jan 2023 23:56:28 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        revest@...omium.org, rostedt@...dmis.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] ftrace: Add sample with custom ops

On Tue,  3 Jan 2023 12:49:12 +0000
Mark Rutland <mark.rutland@....com> wrote:

> When reworking core ftrace code or architectural ftrace code, it's often
> necessary to test/analyse/benchmark a number of ftrace_ops
> configurations. This patch adds a module which can be used to explore
> some of those configurations.
> 
> I'm using this to benchmark various options for changing the way
> trampolines and handling of ftrace_ops work on arm64, and ensuring other
> architectures aren't adversely affected.
> 
> For example, in a QEMU+KVM VM running on a 2GHz Xeon E5-2660
> workstation, loading the module in various configurations produces:
> 
> | # insmod ftrace-ops.ko
> | ftrace_ops: registering:
> |   relevant ops: 1
> |     tracee: tracee_relevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   irrelevant ops: 0
> |     tracee: tracee_irrelevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   saving registers: NO
> |   assist recursion: NO
> |   assist RCU: NO
> | ftrace_ops: Attempted 100000 calls to tracee_relevant [ftrace_ops] in 1681558ns (16ns / call)
> 
> | # insmod ftrace-ops.ko nr_ops_irrelevant=5
> | ftrace_ops: registering:
> |   relevant ops: 1
> |     tracee: tracee_relevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   irrelevant ops: 5
> |     tracee: tracee_irrelevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   saving registers: NO
> |   assist recursion: NO
> |   assist RCU: NO
> | ftrace_ops: Attempted 100000 calls to tracee_relevant [ftrace_ops] in 1693042ns (16ns / call)
> 
> | # insmod ftrace-ops.ko nr_ops_relevant=2
> | ftrace_ops: registering:
> |   relevant ops: 2
> |     tracee: tracee_relevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   irrelevant ops: 0
> |     tracee: tracee_irrelevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   saving registers: NO
> |   assist recursion: NO
> |   assist RCU: NO
> | ftrace_ops: Attempted 100000 calls to tracee_relevant [ftrace_ops] in 11965582ns (119ns / call)
> 
> | # insmod ftrace-ops.ko save_regs=true
> | ftrace_ops: registering:
> |   relevant ops: 1
> |     tracee: tracee_relevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   irrelevant ops: 0
> |     tracee: tracee_irrelevant [ftrace_ops]
> |     tracer: ops_func_nop [ftrace_ops]
> |   saving registers: YES
> |   assist recursion: NO
> |   assist RCU: NO
> | ftrace_ops: Attempted 100000 calls to tracee_relevant [ftrace_ops] in 4459624ns (44ns / call)
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Florent Revest <revest@...omium.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>

Thanks for this useful micro benchmark module :)
This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>


> ---
>  samples/Kconfig             |   7 +
>  samples/Makefile            |   1 +
>  samples/ftrace/Makefile     |   1 +
>  samples/ftrace/ftrace-ops.c | 252 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 261 insertions(+)
>  create mode 100644 samples/ftrace/ftrace-ops.c
> 
> diff --git a/samples/Kconfig b/samples/Kconfig
> index 0d81c00289ee3..daf14c35f071d 100644
> --- a/samples/Kconfig
> +++ b/samples/Kconfig
> @@ -46,6 +46,13 @@ config SAMPLE_FTRACE_DIRECT_MULTI
>  	  that hooks to wake_up_process and schedule, and prints
>  	  the function addresses.
>  
> +config SAMPLE_FTRACE_OPS
> +	tristate "Build custom ftrace ops example"
> +	depends on FUNCTION_TRACER
> +	help
> +	  This builds an ftrace ops example that hooks two functions and
> +	  measures the time taken to invoke one function a number of times.
> +
>  config SAMPLE_TRACE_ARRAY
>          tristate "Build sample module for kernel access to Ftrace instancess"
>  	depends on EVENT_TRACING && m
> diff --git a/samples/Makefile b/samples/Makefile
> index 9832ef3f8fcba..7cb632ef88eeb 100644
> --- a/samples/Makefile
> +++ b/samples/Makefile
> @@ -24,6 +24,7 @@ obj-$(CONFIG_SAMPLE_TRACE_CUSTOM_EVENTS) += trace_events/
>  obj-$(CONFIG_SAMPLE_TRACE_PRINTK)	+= trace_printk/
>  obj-$(CONFIG_SAMPLE_FTRACE_DIRECT)	+= ftrace/
>  obj-$(CONFIG_SAMPLE_FTRACE_DIRECT_MULTI) += ftrace/
> +obj-$(CONFIG_SAMPLE_FTRACE_OPS)		+= ftrace/
>  obj-$(CONFIG_SAMPLE_TRACE_ARRAY)	+= ftrace/
>  subdir-$(CONFIG_SAMPLE_UHID)		+= uhid
>  obj-$(CONFIG_VIDEO_PCI_SKELETON)	+= v4l/
> diff --git a/samples/ftrace/Makefile b/samples/ftrace/Makefile
> index faf8cdb79c5f4..589baf2ec4e3d 100644
> --- a/samples/ftrace/Makefile
> +++ b/samples/ftrace/Makefile
> @@ -5,6 +5,7 @@ obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct-too.o
>  obj-$(CONFIG_SAMPLE_FTRACE_DIRECT) += ftrace-direct-modify.o
>  obj-$(CONFIG_SAMPLE_FTRACE_DIRECT_MULTI) += ftrace-direct-multi.o
>  obj-$(CONFIG_SAMPLE_FTRACE_DIRECT_MULTI) += ftrace-direct-multi-modify.o
> +obj-$(CONFIG_SAMPLE_FTRACE_OPS) += ftrace-ops.o
>  
>  CFLAGS_sample-trace-array.o := -I$(src)
>  obj-$(CONFIG_SAMPLE_TRACE_ARRAY) += sample-trace-array.o
> diff --git a/samples/ftrace/ftrace-ops.c b/samples/ftrace/ftrace-ops.c
> new file mode 100644
> index 0000000000000..24deb51c72618
> --- /dev/null
> +++ b/samples/ftrace/ftrace-ops.c
> @@ -0,0 +1,252 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#define pr_fmt(fmt)       KBUILD_MODNAME ": " fmt
> +
> +#include <linux/ftrace.h>
> +#include <linux/ktime.h>
> +#include <linux/module.h>
> +
> +#include <asm/barrier.h>
> +
> +/*
> + * Arbitrary large value chosen to be sufficiently large to minimize noise but
> + * sufficiently small to complete quickly.
> + */
> +unsigned int nr_function_calls = 100000;
> +module_param(nr_function_calls, uint, 0);
> +MODULE_PARM_DESC(nr_function_calls, "How many times to call the relevant tracee");
> +
> +/*
> + * The number of ops associated with a call site affects whether a tracer can
> + * be called directly or whether it's necessary to go via the list func, which
> + * can be significantly more expensive.
> + */
> +unsigned int nr_ops_relevant = 1;
> +module_param(nr_ops_relevant, uint, 0);
> +MODULE_PARM_DESC(nr_ops_relevant, "How many ftrace_ops to associate with the relevant tracee");
> +
> +/*
> + * On architectures where all call sites share the same trampoline, having
> + * tracers enabled for distinct functions can force the use of the list func
> + * and incur overhead for all call sites.
> + */
> +unsigned int nr_ops_irrelevant = 0;
> +module_param(nr_ops_irrelevant, uint, 0);
> +MODULE_PARM_DESC(nr_ops_irrelevant, "How many ftrace_ops to associate with the irrelevant tracee");
> +
> +/*
> + * On architectures with DYNAMIC_FTRACE_WITH_REGS, saving the full pt_regs can
> + * be more expensive than only saving the minimal necessary regs.
> + */
> +bool save_regs = false;
> +module_param(save_regs, bool, 0);
> +MODULE_PARM_DESC(save_regs, "Register ops with FTRACE_OPS_FL_SAVE_REGS (save all registers in the trampoline)");
> +
> +bool assist_recursion = false;
> +module_param(assist_recursion, bool, 0);
> +MODULE_PARM_DESC(assist_reursion, "Register ops with FTRACE_OPS_FL_RECURSION");
> +
> +bool assist_rcu = false;
> +module_param(assist_rcu, bool, 0);
> +MODULE_PARM_DESC(assist_reursion, "Register ops with FTRACE_OPS_FL_RCU");
> +
> +/*
> + * By default, a trivial tracer is used which immediately returns to mimimize
> + * overhead. Sometimes a consistency check using a more expensive tracer is
> + * desireable.
> + */
> +bool check_count = false;
> +module_param(check_count, bool, 0);
> +MODULE_PARM_DESC(check_count, "Check that tracers are called the expected number of times\n");
> +
> +/*
> + * Usually it's not interesting to leave the ops registered after the test
> + * runs, but sometimes it can be useful to leave them registered so that they
> + * can be inspected through the tracefs 'enabled_functions' file.
> + */
> +bool persist = false;
> +module_param(persist, bool, 0);
> +MODULE_PARM_DESC(persist, "Successfully load module and leave ftrace ops registered after test completes\n");
> +
> +/*
> + * Marked as noinline to ensure that an out-of-line traceable copy is
> + * generated by the compiler.
> + *
> + * The barrier() ensures the compiler won't elide calls by determining there
> + * are no side-effects.
> + */
> +static noinline void tracee_relevant(void)
> +{
> +	barrier();
> +}
> +
> +/*
> + * Marked as noinline to ensure that an out-of-line traceable copy is
> + * generated by the compiler.
> + *
> + * The barrier() ensures the compiler won't elide calls by determining there
> + * are no side-effects.
> + */
> +static noinline void tracee_irrelevant(void)
> +{
> +	barrier();
> +}
> +
> +struct sample_ops {
> +	struct ftrace_ops ops;
> +	unsigned int count;
> +};
> +
> +static void ops_func_nop(unsigned long ip, unsigned long parent_ip,
> +			 struct ftrace_ops *op,
> +			 struct ftrace_regs *fregs)
> +{
> +	/* do nothing */
> +}
> +
> +static void ops_func_count(unsigned long ip, unsigned long parent_ip,
> +			   struct ftrace_ops *op,
> +			   struct ftrace_regs *fregs)
> +{
> +	struct sample_ops *self;
> +
> +	self = container_of(op, struct sample_ops, ops);
> +	self->count++;
> +}
> +
> +struct sample_ops *ops_relevant;
> +struct sample_ops *ops_irrelevant;
> +
> +static struct sample_ops *ops_alloc_init(void *tracee, ftrace_func_t func,
> +					 unsigned long flags, int nr)
> +{
> +	struct sample_ops *ops;
> +
> +	ops = kcalloc(nr, sizeof(*ops), GFP_KERNEL);
> +	if (WARN_ON_ONCE(!ops))
> +		return NULL;
> +
> +	for (unsigned int i = 0; i < nr; i++) {
> +		ops[i].ops.func = func;
> +		ops[i].ops.flags = flags;
> +		WARN_ON_ONCE(ftrace_set_filter_ip(&ops[i].ops, (unsigned long)tracee, 0, 0));
> +		WARN_ON_ONCE(register_ftrace_function(&ops[i].ops));
> +	}
> +
> +	return ops;
> +}
> +
> +static void ops_destroy(struct sample_ops *ops, int nr)
> +{
> +	if (!ops)
> +		return;
> +
> +	for (unsigned int i = 0; i < nr; i++) {
> +		WARN_ON_ONCE(unregister_ftrace_function(&ops[i].ops));
> +		ftrace_free_filter(&ops[i].ops);
> +	}
> +
> +	kfree(ops);
> +}
> +
> +static void ops_check(struct sample_ops *ops, int nr,
> +		      unsigned int expected_count)
> +{
> +	if (!ops || !check_count)
> +		return;
> +
> +	for (unsigned int i = 0; i < nr; i++) {
> +		if (ops->count == expected_count)
> +			continue;
> +		pr_warn("Counter called %u times (expected %u)\n",
> +			ops->count, expected_count);
> +	}
> +}
> +
> +ftrace_func_t tracer_relevant = ops_func_nop;
> +ftrace_func_t tracer_irrelevant = ops_func_nop;
> +
> +static int __init ftrace_ops_sample_init(void)
> +{
> +	unsigned long flags = 0;
> +	ktime_t start, end;
> +	u64 period;
> +
> +	if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) && save_regs) {
> +		pr_info("this kernel does not support saving registers\n");
> +		save_regs = false;
> +	} else if (save_regs) {
> +		flags |= FTRACE_OPS_FL_SAVE_REGS;
> +	}
> +
> +	if (assist_recursion)
> +		flags |= FTRACE_OPS_FL_RECURSION;
> +
> +	if (assist_rcu)
> +		flags |= FTRACE_OPS_FL_RCU;
> +
> +	if (check_count) {
> +		tracer_relevant = ops_func_count;
> +		tracer_irrelevant = ops_func_count;
> +	}
> +
> +	pr_info("registering:\n"
> +		"  relevant ops: %u\n"
> +		"    tracee: %ps\n"
> +		"    tracer: %ps\n"
> +		"  irrelevant ops: %u\n"
> +		"    tracee: %ps\n"
> +		"    tracer: %ps\n"
> +		"  saving registers: %s\n"
> +		"  assist recursion: %s\n"
> +		"  assist RCU: %s\n",
> +		nr_ops_relevant, tracee_relevant, tracer_relevant,
> +		nr_ops_irrelevant, tracee_irrelevant, tracer_irrelevant,
> +		save_regs ? "YES" : "NO",
> +		assist_recursion ? "YES" : "NO",
> +		assist_rcu ? "YES" : "NO");
> +
> +	ops_relevant = ops_alloc_init(tracee_relevant, tracer_relevant,
> +				      flags, nr_ops_relevant);
> +	ops_irrelevant = ops_alloc_init(tracee_irrelevant, tracer_irrelevant,
> +					flags, nr_ops_irrelevant);
> +
> +	start = ktime_get();
> +	for (unsigned int i = 0; i < nr_function_calls; i++)
> +		tracee_relevant();
> +	end = ktime_get();
> +
> +	ops_check(ops_relevant, nr_ops_relevant, nr_function_calls);
> +	ops_check(ops_irrelevant, nr_ops_irrelevant, 0);
> +
> +	period = ktime_to_ns(ktime_sub(end, start));
> +
> +	pr_info("Attempted %u calls to %ps in %lluns (%lluns / call)\n",
> +		nr_function_calls, tracee_relevant,
> +		period, period / nr_function_calls);
> +
> +	if (persist)
> +		return 0;
> +
> +	ops_destroy(ops_relevant, nr_ops_relevant);
> +	ops_destroy(ops_irrelevant, nr_ops_irrelevant);
> +
> +	/*
> +	 * The benchmark completed sucessfully, but there's no reason to keep
> +	 * the module around. Return an error do the user doesn't have to
> +	 * manually unload the module.
> +	 */
> +	return -EINVAL;
> +}
> +module_init(ftrace_ops_sample_init);
> +
> +static void __exit ftrace_ops_sample_exit(void)
> +{
> +	ops_destroy(ops_relevant, nr_ops_relevant);
> +	ops_destroy(ops_irrelevant, nr_ops_irrelevant);
> +}
> +module_exit(ftrace_ops_sample_exit);
> +
> +MODULE_AUTHOR("Mark Rutland");
> +MODULE_DESCRIPTION("Example of using custom ftrace_ops");
> +MODULE_LICENSE("GPL");
> -- 
> 2.30.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>

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