add biosnoop tool

tools/biosnoop.bt

+//
+// biosnoop.bt - basic block I/O tracing tool, showing per I/O latency.
+//               For Linux, uses bpftrace, eBPF.
+//
+// TODO: switch to block tracepoints. Add device, offset, and size columns.
+//
+// 15-Nov-2017	Brendan Gregg	Created this.
+//
+
+BEGIN
+{
+	printf("%-12s %-16s %-6s %7s\n", "TIME(ms)", "COMM", "PID", "LAT(ms)");
+	@epoch = nsecs;
+}
+
+kprobe:blk_account_io_start
+{
+	@start[arg0] = nsecs;
+	@iopid[arg0] = pid;
+	@iocomm[arg0] = comm;
+}
+
+kprobe:blk_account_io_completion
+/@start[arg0] != 0 && @iopid[arg0] != 0 && @iocomm[arg0] != ""/
+
+{
+	$now = nsecs;
+	printf("%-12u %-16s %-6d %7d\n",
+	    ($now - @epoch) / 1000000, @iocomm[arg0], @iopid[arg0],
+	    ($now - @start[arg0]) / 1000000);
+
+	@start[arg0] = delete();
+	@iopid[arg0] = delete();
+	@iocomm[arg0] = delete();
+}
+
+END
+{
+	@epoch = delete();
+}

tools/biosnoop_example.txt

+Demonstrations of biosnoop, the Linux BPF/bpftrace version.
+
+
+This traces block I/O, and shows the issuing process (at least, the process
+that was on-CPU at the time of queue insert) and the latency of the I/O:
+
+# bpftrace biosnoop.bt
+Attaching 4 probes...
+TIME(ms)     COMM             PID    LAT(ms)
+611          bash             4179        10
+611          cksum            4179         0
+627          cksum            4179        15
+641          cksum            4179        13
+644          cksum            4179         3
+658          cksum            4179        13
+673          cksum            4179        14
+686          cksum            4179        13
+701          cksum            4179        14
+710          cksum            4179         8
+717          cksum            4179         6
+728          cksum            4179        10
+735          cksum            4179         6
+751          cksum            4179        10
+758          cksum            4179        17
+783          cksum            4179        12
+796          cksum            4179        25
+802          cksum            4179        32
+[...]
+
+This output shows the cksum process was issuing block I/O, which were
+completing with around 12 milliseconds of latency. Each block I/O event is
+printed out, with a completion time as the first column, measured from
+program start.
+
+
+An example of some background flushing:
+
+# bpftrace biosnoop.bt
+Attaching 4 probes...
+TIME(ms)     COMM             PID    LAT(ms)
+2966         jbd2/nvme0n1-8   615          0
+2967         jbd2/nvme0n1-8   615          0
+[...]