Documentation/libtracefs-cpu-map.txt - pub/scm/libs/libtrace/libtracefs - Git at Google

 libtracefs(3)
 =============

 NAME
 ----
 tracefs_cpu_open_mapped, tracefs_cpu_is_mapped, tracefs_mapped_is_supported, tracefs_cpu_map, tracefs_cpu_unmap - Memory mapping of the ring buffer

 SYNOPSIS
 --------
 [verse]
 --
 *#include <tracefs.h>*

 bool *tracefs_cpu_is_mapped*(struct tracefs_cpu pass:[*]tcpu);
 bool *tracefs_mapped_is_supported*(void);
 int *tracefs_cpu_map*(struct tracefs_cpu pass:[*]tcpu);
 void *tracefs_cpu_unmap*(struct tracefs_cpu pass:[*]tcpu);
 struct tracefs_cpu pass:[*]*tracefs_cpu_open_mapped*(struct tracefs_instance pass:[*]instance,
 					    int cpu, bool nonblock);
 --

 DESCRIPTION
 -----------
 If the trace_pipe_raw supports memory mapping, this is usually a more efficient
 method to stream data from the kernel ring buffer than by reading it, as it does
 not require copying the memory that is being read.

 If memory mapping is supported by the kernel and the application asks to use the
 memory mapping via either *tracefs_cpu_map()* or by *tracefs_cpu_open_mapped()*
 then the functions *tracefs_cpu_read*(3) and *tracefs_cpu_read_buf*(3) will use
 the mapping directly instead of calling the read system call.

 Note, mapping will cause *tracefs_cpu_buffered_read*(3) and *tracefs_cpu_buffered_read_buf*(3)
 to act just like *tracefs_cpu_read*(3) and *tracefs_cpu_read_buf*(3) respectively
 as it doesn't make sense to use a splice pipe when mapped. The kernel will do
 a copy for splice reads on mapping, and then another copy in the function when
 it can avoid the copying if the ring buffer is memory mapped.

 If the _tcpu_ is memory mapped it will also force *tracefs_cpu_write*(3) and
 *tracefs_cpu_pipe*(3) to copy from the mapping instead of using splice.
 Thus care must be used when determining to map the ring buffer or not,
 and why it does not get mapped by default.

 The *tracefs_cpu_is_mapped()* function will return true if _tcpu_ currently has
 its ring buffer memory mapped and false otherwise. This does not return whether or
 not that the kernel supports memory mapping, but that can usually be determined
 by calling *tracefs_cpu_map()*.

 The *tracefs_mapped_is_supported()* returns true if the ring buffer can be
 memory mapped.

 The *tracefs_cpu_map()* function will attempt to map the ring buffer associated
 to _tcpu_ if it is not already mapped.

 The *tracefs_cpu_unmap()* function will unmap the ring buffer associated to
 _tcpu_ if it is mapped.

 The *tracefs_cpu_open_mapped()* is equivalent to calling *tracefs_cpu_open*(3) followed
 by *tracefs_cpu_map()* on the returned _tcpu_ of *tracefs_cpu_open*(3). Note, this
 will still succeed if the mapping fails, in which case it acts the same as
 *tracefs_cpu_open*(3). If knowing if the mapping succeed or not, *tracefs_cpu_is_mapped()*
 should be called on the return _tcpu_.

 RETURN VALUE
 ------------
 *tracefs_cpu_is_mapped()* returns true if the given _tcpu_ has its ring buffer
 memory mapped or false otherwise.

 *tracefs_mapped_is_supported()* returns true if the tracing ring buffer can be
 memory mapped or false if it cannot be or an error occurred.

 *tracefs_cpu_map()* returns 0 on success and -1 on error in mapping. If 0 is
 returned then *tracefs_cpu_is_mapped()* will return true afterward, or false
 if the mapping failed.

 *tracefs_cpu_open_mapped()* returns an allocated tracefs_cpu on success of creation
 regardless if it succeed in mapping the ring buffer or not. It returns NULL for
 the same reasons *tracefs_cpu_open*(3) returns NULL. If success of mapping is
 to be known, then calling *tracefs_cpu_is_mapped()* afterward is required.

 EXAMPLE
 -------
 [source,c]
 --
 #include <stdlib.h>
 #include <ctype.h>
 #include <tracefs.h>

 static void read_subbuf(struct tep_handle *tep, struct kbuffer *kbuf)
 {
 	static struct trace_seq seq;
 	struct tep_record record;
 	int missed_events;

 	if (seq.buffer)
 		trace_seq_reset(&seq);
 	else
 		trace_seq_init(&seq);

 	while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
 		record.size = kbuffer_event_size(kbuf);
 		missed_events = kbuffer_missed_events(kbuf);
 		if (missed_events) {
 			printf("[MISSED EVENTS");
 			if (missed_events > 0)
 				printf(": %d]\n", missed_events);
 			else
 				printf("]\n");
 		}
 		kbuffer_next_event(kbuf, NULL);
 		tep_print_event(tep, &seq, &record,
 				"%s-%d %6.1000d\t%s: %s\n",
 				TEP_PRINT_COMM,
 				TEP_PRINT_PID,
 				TEP_PRINT_TIME,
 				TEP_PRINT_NAME,
 				TEP_PRINT_INFO);
 		trace_seq_do_printf(&seq);
 		trace_seq_reset(&seq);
 	}
 }

 int main (int argc, char **argv)
 {
 	struct tracefs_cpu *tcpu;
 	struct tep_handle *tep;
 	struct kbuffer *kbuf;
 	bool mapped;
 	int cpu;

 	if (argc < 2 || !isdigit(argv[1][0])) {
 		printf("usage: %s cpu\n\n", argv[0]);
 		exit(-1);
 	}

 	cpu = atoi(argv[1]);

 	tep = tracefs_local_events(NULL);
 	if (!tep) {
 		perror("Reading trace event formats");
 		exit(-1);
 	}

 	tcpu = tracefs_cpu_open_mapped(NULL, cpu, 0);
 	if (!tcpu) {
 		perror("Open CPU 0 file");
 		exit(-1);
 	}

 	/*
 	 * If this kernel supports mapping, use normal read,
 	 * otherwise use the piped buffer read, although if
 	 * the mapping succeeded, tracefs_cpu_buffered_read_buf()
 	 * acts the same as tracefs_cpu_read_buf(). But this is just
 	 * an example on how to use tracefs_cpu_is_mapped().
 	 */
 	mapped = tracefs_cpu_is_mapped(tcpu);
 	if (!mapped)
 		printf("Was not able to map, falling back to buffered read\n");
 	while ((kbuf = mapped ? tracefs_cpu_read_buf(tcpu, true) :
 			tracefs_cpu_buffered_read_buf(tcpu, true))) {
 		read_subbuf(tep, kbuf);
 	}

 	kbuf = tracefs_cpu_flush_buf(tcpu);
 	if (kbuf)
 		read_subbuf(tep, kbuf);

 	tracefs_cpu_close(tcpu);
 	tep_free(tep);

 	return 0;
 }
 --

 FILES
 -----
 [verse]
 --
 *tracefs.h*
 	Header file to include in order to have access to the library APIs.
 *-ltracefs*
 	Linker switch to add when building a program that uses the library.
 --

 SEE ALSO
 --------
 *tracefs_cpu_open*(3),
 *tracefs_cpu_read*(3),
 *tracefs_cpu_read_buf*(3),
 *tracefs_cpu_buffered_read*(3),
 *tracefs_cpu_buffered_read_buf*(3),
 *libtracefs*(3),
 *libtraceevent*(3),
 *trace-cmd*(1)

 AUTHOR
 ------
 [verse]
 --
 *Steven Rostedt* <rostedt@goodmis.org>
 --
 REPORTING BUGS
 --------------
 Report bugs to  <linux-trace-devel@vger.kernel.org>

 LICENSE
 -------
 libtracefs is Free Software licensed under the GNU LGPL 2.1

 RESOURCES
 ---------
 https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

 COPYING
 -------
 Copyright \(C) 2022 Google, Inc. Free use of this software is granted under
 the terms of the GNU Public License (GPL).
	libtracefs(3)
	=============

	NAME
	----
	tracefs_cpu_open_mapped, tracefs_cpu_is_mapped, tracefs_mapped_is_supported, tracefs_cpu_map, tracefs_cpu_unmap - Memory mapping of the ring buffer

	SYNOPSIS
	--------
	[verse]
	--
	#include <tracefs.h>

	bool tracefs_cpu_is_mapped(struct tracefs_cpu pass:[*]tcpu);
	bool tracefs_mapped_is_supported(void);
	int tracefs_cpu_map(struct tracefs_cpu pass:[*]tcpu);
	void tracefs_cpu_unmap(struct tracefs_cpu pass:[*]tcpu);
	struct tracefs_cpu pass:[]tracefs_cpu_open_mapped(struct tracefs_instance pass:[]instance,
	int cpu, bool nonblock);
	--

	DESCRIPTION
	-----------
	If the trace_pipe_raw supports memory mapping, this is usually a more efficient
	method to stream data from the kernel ring buffer than by reading it, as it does
	not require copying the memory that is being read.

	If memory mapping is supported by the kernel and the application asks to use the
	memory mapping via either tracefs_cpu_map() or by tracefs_cpu_open_mapped()
	then the functions tracefs_cpu_read(3) and tracefs_cpu_read_buf(3) will use
	the mapping directly instead of calling the read system call.

	Note, mapping will cause tracefs_cpu_buffered_read(3) and tracefs_cpu_buffered_read_buf(3)
	to act just like tracefs_cpu_read(3) and tracefs_cpu_read_buf(3) respectively
	as it doesn't make sense to use a splice pipe when mapped. The kernel will do
	a copy for splice reads on mapping, and then another copy in the function when
	it can avoid the copying if the ring buffer is memory mapped.

	If the _tcpu_ is memory mapped it will also force tracefs_cpu_write(3) and
	tracefs_cpu_pipe(3) to copy from the mapping instead of using splice.
	Thus care must be used when determining to map the ring buffer or not,
	and why it does not get mapped by default.

	The tracefs_cpu_is_mapped() function will return true if _tcpu_ currently has
	its ring buffer memory mapped and false otherwise. This does not return whether or
	not that the kernel supports memory mapping, but that can usually be determined
	by calling tracefs_cpu_map().

	The tracefs_mapped_is_supported() returns true if the ring buffer can be
	memory mapped.

	The tracefs_cpu_map() function will attempt to map the ring buffer associated
	to _tcpu_ if it is not already mapped.

	The tracefs_cpu_unmap() function will unmap the ring buffer associated to
	_tcpu_ if it is mapped.

	The tracefs_cpu_open_mapped() is equivalent to calling tracefs_cpu_open(3) followed
	by tracefs_cpu_map() on the returned _tcpu_ of tracefs_cpu_open(3). Note, this
	will still succeed if the mapping fails, in which case it acts the same as
	tracefs_cpu_open(3). If knowing if the mapping succeed or not, tracefs_cpu_is_mapped()
	should be called on the return _tcpu_.

	RETURN VALUE
	------------
	tracefs_cpu_is_mapped() returns true if the given _tcpu_ has its ring buffer
	memory mapped or false otherwise.

	tracefs_mapped_is_supported() returns true if the tracing ring buffer can be
	memory mapped or false if it cannot be or an error occurred.

	tracefs_cpu_map() returns 0 on success and -1 on error in mapping. If 0 is
	returned then tracefs_cpu_is_mapped() will return true afterward, or false
	if the mapping failed.

	tracefs_cpu_open_mapped() returns an allocated tracefs_cpu on success of creation
	regardless if it succeed in mapping the ring buffer or not. It returns NULL for
	the same reasons tracefs_cpu_open(3) returns NULL. If success of mapping is
	to be known, then calling tracefs_cpu_is_mapped() afterward is required.

	EXAMPLE
	-------
	[source,c]
	--
	#include <stdlib.h>
	#include <ctype.h>
	#include <tracefs.h>

	static void read_subbuf(struct tep_handle tep, struct kbuffer kbuf)
	{
	static struct trace_seq seq;
	struct tep_record record;
	int missed_events;

	if (seq.buffer)
	trace_seq_reset(&seq);
	else
	trace_seq_init(&seq);

	while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
	record.size = kbuffer_event_size(kbuf);
	missed_events = kbuffer_missed_events(kbuf);
	if (missed_events) {
	printf("[MISSED EVENTS");
	if (missed_events > 0)
	printf(": %d]\n", missed_events);
	else
	printf("]\n");
	}
	kbuffer_next_event(kbuf, NULL);
	tep_print_event(tep, &seq, &record,
	"%s-%d %6.1000d\t%s: %s\n",
	TEP_PRINT_COMM,
	TEP_PRINT_PID,
	TEP_PRINT_TIME,
	TEP_PRINT_NAME,
	TEP_PRINT_INFO);
	trace_seq_do_printf(&seq);
	trace_seq_reset(&seq);
	}
	}

	int main (int argc, char **argv)
	{
	struct tracefs_cpu *tcpu;
	struct tep_handle *tep;
	struct kbuffer *kbuf;
	bool mapped;
	int cpu;

	if (argc < 2 \|\| !isdigit(argv[1][0])) {
	printf("usage: %s cpu\n\n", argv[0]);
	exit(-1);
	}

	cpu = atoi(argv[1]);

	tep = tracefs_local_events(NULL);
	if (!tep) {
	perror("Reading trace event formats");
	exit(-1);
	}

	tcpu = tracefs_cpu_open_mapped(NULL, cpu, 0);
	if (!tcpu) {
	perror("Open CPU 0 file");
	exit(-1);
	}

	/*
	* If this kernel supports mapping, use normal read,
	* otherwise use the piped buffer read, although if
	* the mapping succeeded, tracefs_cpu_buffered_read_buf()
	* acts the same as tracefs_cpu_read_buf(). But this is just
	* an example on how to use tracefs_cpu_is_mapped().
	*/
	mapped = tracefs_cpu_is_mapped(tcpu);
	if (!mapped)
	printf("Was not able to map, falling back to buffered read\n");
	while ((kbuf = mapped ? tracefs_cpu_read_buf(tcpu, true) :
	tracefs_cpu_buffered_read_buf(tcpu, true))) {
	read_subbuf(tep, kbuf);
	}

	kbuf = tracefs_cpu_flush_buf(tcpu);
	if (kbuf)
	read_subbuf(tep, kbuf);

	tracefs_cpu_close(tcpu);
	tep_free(tep);

	return 0;
	}
	--

	FILES
	-----
	[verse]
	--
	tracefs.h
	Header file to include in order to have access to the library APIs.
	-ltracefs
	Linker switch to add when building a program that uses the library.
	--

	SEE ALSO
	--------
	tracefs_cpu_open(3),
	tracefs_cpu_read(3),
	tracefs_cpu_read_buf(3),
	tracefs_cpu_buffered_read(3),
	tracefs_cpu_buffered_read_buf(3),
	libtracefs(3),
	libtraceevent(3),
	trace-cmd(1)

	AUTHOR
	------
	[verse]
	--
	Steven Rostedt <rostedt@goodmis.org>
	--
	REPORTING BUGS
	--------------
	Report bugs to <linux-trace-devel@vger.kernel.org>

	LICENSE
	-------
	libtracefs is Free Software licensed under the GNU LGPL 2.1

	RESOURCES
	---------
	https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

	COPYING
	-------
	Copyright \(C) 2022 Google, Inc. Free use of this software is granted under
	the terms of the GNU Public License (GPL).