@@ -360,6 +360,13 @@ OPTIONS
executed will not be changed. This is useful if you want to monitor the
output of the command being executed, but not see the output from trace-cmd.
+*--poll*::
+ Waiting for data to be available on the trace ring-buffers may trigger
+ IPIs. This might generate unacceptable trace noise when tracing low latency
+ or real time systems. The poll option forces trace-cmd to use O_NONBLOCK.
+ Traces are extracted by busy waiting, which will hog the CPUs, so only use
+ when really needed.
+
EXAMPLES
--------
@@ -319,8 +319,9 @@ struct tracecmd_output *tracecmd_get_output_handle_fd(int fd);
enum {
TRACECMD_RECORD_NOSPLICE = (1 << 0), /* Use read instead of splice */
TRACECMD_RECORD_SNAPSHOT = (1 << 1), /* Extract from snapshot */
- TRACECMD_RECORD_BLOCK = (1 << 2), /* Block on splice write */
+ TRACECMD_RECORD_BLOCK_SPLICE = (1 << 2), /* Block on splice write */
TRACECMD_RECORD_NOBRASS = (1 << 3), /* Splice directly without a brass pipe */
+ TRACECMD_RECORD_POLL = (1 << 4), /* Use O_NONBLOCK, poll trace buffers */
};
void tracecmd_free_recorder(struct tracecmd_recorder *recorder);
@@ -112,6 +112,18 @@ void tracecmd_free_recorder(struct tracecmd_recorder *recorder)
free(recorder);
}
+static void set_nonblock(struct tracecmd_recorder *recorder)
+{
+ long flags;
+
+ /* Do not block on reads */
+ flags = fcntl(recorder->trace_fd, F_GETFL);
+ fcntl(recorder->trace_fd, F_SETFL, flags | O_NONBLOCK);
+
+ /* Do not block on streams */
+ recorder->fd_flags |= SPLICE_F_NONBLOCK;
+}
+
struct tracecmd_recorder *
tracecmd_create_buffer_recorder_fd2(int fd, int fd2, int cpu, unsigned flags,
const char *buffer, int maxkb)
@@ -130,7 +142,7 @@ tracecmd_create_buffer_recorder_fd2(int fd, int fd2, int cpu, unsigned flags,
recorder->fd_flags = SPLICE_F_MOVE;
- if (!(recorder->flags & TRACECMD_RECORD_BLOCK))
+ if (!(recorder->flags & TRACECMD_RECORD_BLOCK_SPLICE))
recorder->fd_flags |= SPLICE_F_NONBLOCK;
recorder->trace_fd_flags = SPLICE_F_MOVE;
@@ -198,6 +210,9 @@ tracecmd_create_buffer_recorder_fd2(int fd, int fd2, int cpu, unsigned flags,
recorder->pipe_size = pipe_size;
}
+ if (recorder->flags & TRACECMD_RECORD_POLL)
+ set_nonblock(recorder);
+
return recorder;
out_free:
@@ -504,18 +519,6 @@ static long move_data(struct tracecmd_recorder *recorder)
return splice_data(recorder);
}
-static void set_nonblock(struct tracecmd_recorder *recorder)
-{
- long flags;
-
- /* Do not block on reads for flushing */
- flags = fcntl(recorder->trace_fd, F_GETFL);
- fcntl(recorder->trace_fd, F_SETFL, flags | O_NONBLOCK);
-
- /* Do not block on streams for write */
- recorder->fd_flags |= SPLICE_F_NONBLOCK;
-}
-
long tracecmd_flush_recording(struct tracecmd_recorder *recorder)
{
char buf[recorder->page_size];
@@ -3333,7 +3333,7 @@ create_recorder_instance_pipe(struct buffer_instance *instance,
int cpu, int *brass)
{
struct tracecmd_recorder *recorder;
- unsigned flags = recorder_flags | TRACECMD_RECORD_BLOCK;
+ unsigned flags = recorder_flags | TRACECMD_RECORD_BLOCK_SPLICE;
char *path;
path = tracefs_instance_get_dir(instance->tracefs);
@@ -5757,6 +5757,7 @@ enum {
OPT_module = 256,
OPT_nofifos = 257,
OPT_cmdlines_size = 258,
+ OPT_poll = 259,
};
void trace_stop(int argc, char **argv)
@@ -6172,6 +6173,7 @@ static void parse_record_options(int argc,
{"tsync-interval", required_argument, NULL, OPT_tsyncinterval},
{"fork", no_argument, NULL, OPT_fork},
{"tsc2nsec", no_argument, NULL, OPT_tsc2nsec},
+ {"poll", no_argument, NULL, OPT_poll},
{NULL, 0, NULL, 0}
};
@@ -6593,6 +6595,10 @@ static void parse_record_options(int argc,
die("TSC to nanosecond is not supported");
ctx->instance->flags |= BUFFER_FL_TSC2NSEC;
break;
+ case OPT_poll:
+ cmd_check_die(ctx, CMD_set, *(argv+1), "--poll");
+ recorder_flags |= TRACECMD_RECORD_POLL;
+ break;
case OPT_quiet:
case 'q':
quiet = true;
@@ -68,6 +68,7 @@ static struct usage_help usage_help[] = {
" If a negative number is specified, timestamps synchronization is disabled"
" If 0 is specified, no loop is performed - timestamps offset is calculated only twice,"
" at the beginnig and at the end of the trace\n"
+ " --poll don't block while reading from the trace buffer\n"
},
{
"set",
Waiting for data to be available on the trace ring-buffers may trigger IPIs. This might generate unacceptable trace noise when debugging low latency or real time systems. So introduce the poll option. When enabled, it forces trace-cmd to use O_NONBLOCK. The drawback to using it is that traces will be extracted by busy waiting, which will unnecessarily hog the CPUs, so only use when really needed. Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> --- NOTE: I'm aware that TRACECMD_RECORD_POLL kind of clashes with TRACECMD_RECORD_BLOCK. That's why I renamed it. There might be a clean way to merge them into a single flag, but it's not clear to me why was TRACECMD_RECORD_BLOCK needed in the first place (since it only affects splicing). .../trace-cmd/trace-cmd-record.1.txt | 7 +++++ .../include/private/trace-cmd-private.h | 3 +- lib/trace-cmd/trace-recorder.c | 29 ++++++++++--------- tracecmd/trace-record.c | 8 ++++- tracecmd/trace-usage.c | 1 + 5 files changed, 33 insertions(+), 15 deletions(-)