perf_counter tools: Reduce perf stat measurement overhead/skew

[deliverable/linux.git] / tools / perf / builtin-stat.c

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index c5a290727a92f8b529e8fc81a28455f2c77ac386..201ef2367dcbf515be730c48fadde74118fc02e6 100644 (file)
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -99,7 +99,7 @@ static u64                    runtime_cycles_noise;
 #define ERR_PERF_OPEN \
 "Error: counter %d, sys_perf_counter_open() syscall returned with %d (%s)\n"
 
-static void create_perf_stat_counter(int counter)
+static void create_perf_stat_counter(int counter, int pid)
 {
        struct perf_counter_attr *attr = attrs + counter;
 
@@ -119,7 +119,7 @@ static void create_perf_stat_counter(int counter)
                attr->inherit   = inherit;
                attr->disabled  = 1;
 
-               fd[0][counter] = sys_perf_counter_open(attr, 0, -1, -1, 0);
+               fd[0][counter] = sys_perf_counter_open(attr, pid, -1, -1, 0);
                if (fd[0][counter] < 0 && verbose)
                        fprintf(stderr, ERR_PERF_OPEN, counter,
                                fd[0][counter], strerror(errno));
@@ -205,12 +205,58 @@ static int run_perf_stat(int argc, const char **argv)
        int status = 0;
        int counter;
        int pid;
+       int child_ready_pipe[2], go_pipe[2];
+       char buf;
 
        if (!system_wide)
                nr_cpus = 1;
 
+       if (pipe(child_ready_pipe) < 0 || pipe(go_pipe) < 0) {
+               perror("failed to create pipes");
+               exit(1);
+       }
+
+       if ((pid = fork()) < 0)
+               perror("failed to fork");
+
+       if (!pid) {
+               close(child_ready_pipe[0]);
+               close(go_pipe[1]);
+               fcntl(go_pipe[0], F_SETFD, FD_CLOEXEC);
+
+               /*
+                * Do a dummy execvp to get the PLT entry resolved,
+                * so we avoid the resolver overhead on the real
+                * execvp call.
+                */
+               execvp("", (char **)argv);
+
+               /*
+                * Tell the parent we're ready to go
+                */
+               close(child_ready_pipe[1]);
+
+               /*
+                * Wait until the parent tells us to go.
+                */
+               read(go_pipe[0], &buf, 1);
+
+               execvp(argv[0], (char **)argv);
+
+               perror(argv[0]);
+               exit(-1);
+       }
+
+       /*
+        * Wait for the child to be ready to exec.
+        */
+       close(child_ready_pipe[1]);
+       close(go_pipe[0]);
+       read(child_ready_pipe[0], &buf, 1);
+       close(child_ready_pipe[0]);
+
        for (counter = 0; counter < nr_counters; counter++)
-               create_perf_stat_counter(counter);
+               create_perf_stat_counter(counter, pid);
 
        /*
         * Enable counters and exec the command:
@@ -218,19 +264,9 @@ static int run_perf_stat(int argc, const char **argv)
        t0 = rdclock();
        prctl(PR_TASK_PERF_COUNTERS_ENABLE);
 
-       if ((pid = fork()) < 0)
-               perror("failed to fork");
-
-       if (!pid) {
-               if (execvp(argv[0], (char **)argv)) {
-                       perror(argv[0]);
-                       exit(-1);
-               }
-       }
-
+       close(go_pipe[1]);
        wait(&status);
 
-       prctl(PR_TASK_PERF_COUNTERS_DISABLE);
        t1 = rdclock();
 
        walltime_nsecs[run_idx] = t1 - t0;