trace-cmd: Add '-r' to record to set priority of capture threads

Add the option '-r' to the trace-cmd record that will set the
trace capture threads to that real-time FIFO priority.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

3 files changed, 29 insertions, 2 deletions

diff --git a/Documentation/trace-cmd-record.1.txt b/Documentation/trace-cmd-record.1.txt
index 4707605..4e09389 100644
--- a/Documentation/trace-cmd-record.1.txt
+++ b/Documentation/trace-cmd-record.1.txt
@@ -125,6 +125,13 @@ OPTIONS
     This is the time each recording process will sleep before waking up to
     record any new data that was written to the ring buffer.
 
+*-r* 'priority'::
+    The priority to run the capture threads at. In a busy system the trace
+    capturing threads may be staved and events can be lost. This increases
+    the priority of those threads to the real time (FIFO) priority.
+    But use this option with care, it can also change the behaviour of
+    the system being traced.
+
 *-b* 'size'::
     This sets the ring buffer size to 'size' kilobytes. Because the Ftrace
     ring buffer is per CPU, this size is the size of each per CPU ring buffer
diff --git a/trace-cmd.c b/trace-cmd.c
index 4db8e8c..7c13264 100644
--- a/trace-cmd.c
+++ b/trace-cmd.c
@@ -33,6 +33,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <ctype.h>
+#include <sched.h>
 #include <errno.h>
 #include <glob.h>
 
@@ -54,6 +55,8 @@
 int silence_warnings;
 int show_status;
 
+static int rt_prio;
+
 static int use_tcp;
 
 static unsigned int page_size;
@@ -963,6 +966,16 @@ static void connect_port(int cpu)
         client_ports[cpu] = sfd;
 }
 
+static void set_prio(int prio)
+{
+        struct sched_param sp;
+
+        memset(&sp, 0, sizeof(sp));
+        sp.sched_priority = prio;
+        if (sched_setscheduler(0, SCHED_FIFO, &sp) < 0)
+                warning("failed to set priority");
+}
+
 static int create_recorder(int cpu)
 {
         char *file;
@@ -978,6 +991,9 @@ static int create_recorder(int cpu)
         signal(SIGINT, finish);
         signal(SIGUSR1, flush);
 
+        if (rt_prio)
+                set_prio(rt_prio);
+
         /* do not kill tasks on error */
         cpu_count = 0;
 
@@ -1325,7 +1341,7 @@ int main (int argc, char **argv)
                    (strcmp(argv[1], "start") == 0) ||
                    ((extract = strcmp(argv[1], "extract") == 0))) {
 
-                while ((c = getopt(argc-1, argv+1, "+he:f:Fp:do:O:s:vg:l:n:P:N:tb:")) >= 0) {
+                while ((c = getopt(argc-1, argv+1, "+he:f:Fp:do:O:s:r:vg:l:n:P:N:tb:")) >= 0) {
                         switch (c) {
                         case 'h':
                                 usage(argv);
@@ -1418,6 +1434,9 @@ int main (int argc, char **argv)
                                         usage(argv);
                                 sleep_time = atoi(optarg);
                                 break;
+                        case 'r':
+                                rt_prio = atoi(optarg);
+                                break;
                         case 'N':
                                 if (!record)
                                         die("-N only available with record");
diff --git a/trace-usage.c b/trace-usage.c
index e834d44..9df19eb 100644
--- a/trace-usage.c
+++ b/trace-usage.c
@@ -18,7 +18,7 @@ static struct usage_help usage_help[] = {
                 "record a trace into a trace.dat file",
                 " %s record [-v][-e event [-f filter]][-p plugin][-F][-d][-o file] \\\n"
                 "           [-s usecs][-O option ][-l func][-g func][-n func] \\\n"
-                "           [-P pid][-N host:port][-t][-b size][command ...]\n"
+                "           [-P pid][-N host:port][-t][-r prio][-b size][command ...]\n"
                 "          -e run command with event enabled\n"
                 "          -f filter for previous -e event\n"
                 "          -p run command with plugin enabled\n"
@@ -31,6 +31,7 @@ static struct usage_help usage_help[] = {
                 "          -d disable function tracer when running\n"
                 "          -o data output file [default trace.dat]\n"
                 "          -O option to enable (or disable)\n"
+                "          -r real time priority to run the capture threads\n"
                 "          -s sleep interval between recording (in usecs) [default: 1000]\n"
                 "          -N host:port to connect to (see listen)\n"
                 "          -t used with -N, forces use of tcp in live trace\n"