perf script: Support relative time

When comparing time stamps in 'perf script' traces it can be annoying to
work with the full perf time stamps.

Add a --reltime option that displays time stamps relative to the trace
start to make it easier to read the traces.

Note: not currently supported for --time. Report an error in this
case.

Before:

% perf script
swapper 0 [000] 245402.891216: 1 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 245402.891223: 1 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 245402.891227: 5 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 245402.891231: 41 cycles:ppp: ffffffffa0068816 native_write_msr+0x6 ([kernel.kallsyms])
swapper 0 [000] 245402.891235: 355 cycles:ppp: ffffffffa000dd51 intel_bts_enable_local+0x21 ([kernel.kallsyms])
swapper 0 [000] 245402.891239: 3084 cycles:ppp: ffffffffa0a0150a end_repeat_nmi+0x48 ([kernel.kallsyms])

After:

% perf script --reltime

swapper 0 [000] 0.000000: 1 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 0.000006: 1 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 0.000010: 5 cycles:ppp: ffffffffa0068814 native_write_msr+0x4 ([kernel.kallsyms])
swapper 0 [000] 0.000014: 41 cycles:ppp: ffffffffa0068816 native_write_msr+0x6 ([kernel.kallsyms])
swapper 0 [000] 0.000018: 355 cycles:ppp: ffffffffa000dd51 intel_bts_enable_local+0x21 ([kernel.kallsyms])
swapper 0 [000] 0.000022: 3084 cycles:ppp: ffffffffa0a0150a end_repeat_nmi+0x48 ([kernel.kallsyms])

Committer notes:

Do not use 'time' as the name of a variable, as this breaks the build on
older glibcs:

cc1: warnings being treated as errors
builtin-script.c: In function 'perf_sample__fprintf_start':
builtin-script.c:691: warning: declaration of 'time' shadows a global declaration
/usr/include/time.h:187: warning: shadowed declaration is here

Signed-off-by: Andi Kleen <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
LPU-Reference: [e-mail]
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-script.txt

···
 	Set the maximum number of program blocks to print with brstackasm for
 	each sample.
 
+--reltime::
+	Print time stamps relative to trace start.
+
 --per-event-dump::
 	Create per event files with a "perf.data.EVENT.dump" name instead of
         printing to stdout, useful, for instance, for generating flamegraphs.

tools/perf/builtin-script.c

···
 
 static char const		*script_name;
 static char const		*generate_script_lang;
+static bool			reltime;
+static u64			initial_time;
 static bool			debug_mode;
 static u64			last_timestamp;
 static u64			nr_unordered;
···
 	}
 
 	if (PRINT_FIELD(TIME)) {
-		nsecs = sample->time;
+		u64 t = sample->time;
+		if (reltime) {
+			if (!initial_time)
+				initial_time = sample->time;
+			t = sample->time - initial_time;
+		}
+		nsecs = t;
 		secs = nsecs / NSEC_PER_SEC;
 		nsecs -= secs * NSEC_PER_SEC;
 
···
 			printed += fprintf(fp, "%5lu.%09llu: ", secs, nsecs);
 		else {
 			char sample_time[32];
-			timestamp__scnprintf_usec(sample->time, sample_time, sizeof(sample_time));
+			timestamp__scnprintf_usec(t, sample_time, sizeof(sample_time));
 			printed += fprintf(fp, "%12s: ", sample_time);
 		}
 	}
···
 		     "Set the maximum stack depth when parsing the callchain, "
 		     "anything beyond the specified depth will be ignored. "
 		     "Default: kernel.perf_event_max_stack or " __stringify(PERF_MAX_STACK_DEPTH)),
+	OPT_BOOLEAN(0, "reltime", &reltime, "Show time stamps relative to start"),
 	OPT_BOOLEAN('I', "show-info", &show_full_info,
 		    "display extended information from perf.data file"),
 	OPT_BOOLEAN('\0', "show-kernel-path", &symbol_conf.show_kernel_path,
···
 				"(see 'perf script -l' for listing)\n");
 			return -1;
 		}
+	}
+
+	if (script.time_str && reltime) {
+		fprintf(stderr, "Don't combine --reltime with --time\n");
+		return -1;
 	}
 
 	if (itrace_synth_opts.callchain &&