Add an example program to show how to use libpmc from a program.

This particular program attempts to use the TSC to measure how long
certainly libpmc operations take.  Depending on the quality of
the rdtsc() macro on a particular architecture this may work
more or less well.

share/examples/hwpmc/Makefile

@@ -0,0 +1,9 @@
+# $FreeBSD$
+
+PROG=	overhead
+LDFLAGS+= -lpmc
+MAN=
+
+install:
+
+.include <bsd.prog.mk>

share/examples/hwpmc/README

@@ -3,6 +3,4 @@
 Examples illustrating the use of the hwpmc(4) driver and pmc(3)
 library interface.
 
-While there is nothing here yet, the source code for pmccontrol(8)
-and pmcstat(8) could serve as examples.
 

share/examples/hwpmc/overhead.c

@@ -0,0 +1,115 @@
+/* 
+ * Copyright (c) 2014, Neville-Neil Consulting
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ * 
+ * Redistributions of source code must retain the above copyright notice,
+ * this list of conditions and the following disclaimer.
+ * 
+ * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ * 
+ * Neither the name of Neville-Neil Consulting nor the names of its 
+ * contributors may be used to endorse or promote products derived from 
+ * this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ *
+ * Author: George V. Neville-Neil
+ *
+ *
+ */
+
+#ifndef lint
+static char rcsid [] = "$Header:$" ;
+#endif
+
+/*
+ * Calculate the time overhead of starting, stopping, and recording
+ * pmc counters.
+ *
+ * The only argument is a counter name, such as "instruction-retired"
+ * which is CPU dependent and can be found with pmmcontrol(8) using
+ * pmccontrol -L.
+ *
+ * The start, stop, read and write operations are timed using the
+ * rdtsc() macro which reads the Time Stamp Counter on the CPU.
+ */
+
+#include <stdio.h>
+#include <err.h>
+#include <sysexits.h>
+#include <sys/types.h>
+#include <machine/cpufunc.h>
+#include <pmc.h>
+
+int main(int argc, char **argv)
+{
+	pmc_id_t pmcid;
+	pmc_value_t read_value;
+	pmc_value_t read_clear_value;
+	uint64_t tsc1, write_cyc, start_cyc, read_cyc, stop_cyc;
+	char *counter_name;
+	
+	if (argc != 2)
+		err(EX_USAGE, "counter-name required");
+
+	counter_name = argv[1];
+	
+	if (pmc_init() != 0)
+		err(EX_OSERR, "hwpmc(4) not loaded, kldload or update your kernel");
+
+	if (pmc_allocate(counter_name, PMC_MODE_SC, 0, 0, &pmcid) < 0)
+		err(EX_OSERR, "failed to allocate %s as a system counter in counting mode",
+		    counter_name);
+
+	tsc1 = rdtsc();
+	if (pmc_write(pmcid, 0) < 0)
+		err(EX_OSERR, "failed to zero counter %s", counter_name);
+	write_cyc = rdtsc() - tsc1;
+
+	tsc1 = rdtsc();
+	if (pmc_start(pmcid) < 0)
+		err(EX_OSERR, "failed to start counter %s", counter_name);
+	start_cyc = rdtsc() - tsc1;
+
+	tsc1 = rdtsc();
+	if (pmc_read(pmcid, &read_value) < 0)
+		err(EX_OSERR, "failed to read counter %s", counter_name);
+	read_cyc = rdtsc() - tsc1;
+
+	tsc1 = rdtsc();
+	if (pmc_stop(pmcid) < 0)
+		err(EX_OSERR, "failed to stop counter %s", counter_name);
+	stop_cyc = rdtsc() - tsc1;
+	
+	if (pmc_rw(pmcid, 0, &read_clear_value))
+		err(EX_OSERR, "failed to read and zero %s", counter_name);
+
+	if (pmc_release(pmcid) < 0)
+		err(EX_OSERR, "failed to release %s as a system counter in counting mode",
+		    counter_name);
+
+	printf("Counter %s, read value %ld, read/clear value %ld\n",
+	    counter_name, read_value, read_clear_value);
+	printf("Cycles to start: %ld\tstop: %ld\tread: %ld\twrite: %ld\n",
+	    start_cyc, stop_cyc, read_cyc, stop_cyc);
+
+}
+