Add bpftrace examples to the demo

bpftrace is at least as prevalent as bcc-tools now so lets include it in
the example commands to be used with the demo.

README.md

@@ -50,10 +50,19 @@ anything after that. In the second window run the bcc trace program on the
 `salpdemo` process, monitoring probes `p1` and `p2`.
 
 ```bash
-sudo trace -p "$(pgrep salpdemo | head -n1)" \
-    'u::p1 "arg1=%d arg2=%s", arg1, arg2' \
-    'u::p2 "arg1=%d", arg1'
+sudo trace -p "$(pgrep -n salpdemo)"                    \
+    'u::p1 "i=%d err='%s' date='%s'", arg1, arg2, arg3' \
+    'u::p2 "j=%d flag=%d", arg1, arg2'
 ```
 
-`trace` will output the values of `arg1` and `arg2` from probe `p1` and the
-value of `arg1` from probe `p2`.
+or alternatively the same thing with `bpftrace`
+
+```bash
+sudo bpftrace -p "$(pgrep -n salpdemo)" /dev/stdin <<EOF
+  usdt:p1 { printf("i=%d err='%s' date='%s'\n", arg0, str(arg1), str(arg2)); }
+  usdt:p2 { printf("j=%d flag=%d\n", arg0, arg1); }
+EOF
+```
+
+Either trace invocations will output the values of the three args to probe 1 and
+the two args to probe 2.

internal/salpdemo.go

@@ -18,12 +18,20 @@ var (
 
 func main() {
 	defer salp.UnloadAndDispose(probes)
-	fmt.Println("List the go probes in this demo with")
-	fmt.Println("\tsudo tplist -vp \"$(pgrep salpdemo)\" \"salp-demo*\"")
-	fmt.Println("Trace this process with")
-	fmt.Println("\tsudo trace -p \"$(pgrep salpdemo | head -n1)\" 'u::p1 \"i=%d err=`%s` date=`%s`\", arg1, arg2, arg3' 'u::p2 \"j=%d flag=%d\", arg1, arg2'")
-	fmt.Println("\tor")
-	fmt.Println("\tsudo trace -p \"$(pgrep salpdemo | head -n1)\" 'u::p1 (arg1 % 2 == 0) \"i=%d err='%s'\", arg1, arg2'")
+	fmt.Println(`List the go probes in this demo
+	# With bcc tools
+	sudo tplist -vp "$(pgrep -n salpdemo)" "*salp-demo*"
+
+	# With bpftrace
+	sudo bpftrace -p "$(pgrep -n salpdemo)" -l "usdt:*:salp-demo:*"
+
+Trace this process
+	# With bcc tools
+	sudo trace -p "$(pgrep -n salpdemo)" 'u::p1 "i=%d err='%s' date='%s'", arg1, arg2, arg3' 'u::p2 "j=%d flag=%d", arg1, arg2'
+
+	# With bpftrace
+	sudo bpftrace -p "$(pgrep -n salpdemo)" -e 'usdt:p1 { printf("%d (%s)\n", arg0, str(arg2)); }'
+	sudo bpftrace -p "$(pgrep -n salpdemo)" -e 'usdt:p2 { if(arg1) { printf("Truthy! %d\n", arg0); } }'`)
 
 	salp.MustLoadProvider(probes)