Provided by: libtracefs-doc_1.8.0-1ubuntu1_all 
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
#include <tracefs.h>
bool tracefs_cpu_is_mapped(struct tracefs_cpu *tcpu);
bool tracefs_mapped_is_supported(void);
int tracefs_cpu_map(struct tracefs_cpu *tcpu);
void tracefs_cpu_unmap(struct tracefs_cpu *tcpu);
struct tracefs_cpu *tracefs_cpu_open_mapped(struct tracefs_instance *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
#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
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
Steven Rostedt <rostedt@goodmis.org[1]>
REPORTING BUGS
Report bugs to <linux-trace-devel@vger.kernel.org[2]>
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).
NOTES
1. rostedt@goodmis.org
mailto:rostedt@goodmis.org
2. linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org