perf stat: Reduce min --interval-print to 10ms

The --interval-print parameter was limited to 100ms. However, for
example, 10ms is required to do sophisticated bandwidth analysis using
uncore events.

The test shows that the overhead of the system-wide uncore monitoring
with 10ms interval is only ~2%. So this patch reduces the minimal
interval-print allowd to 10ms.

But 10ms may not work well for all cases. For example, when the
cpus/threads number is very large, for system-wide core event monitoring
the overhead could be high.

To handle this issue, a warning will be displayed when the
interval-print is set between 10ms to 100ms. So users can make a
decision according to their specific cases.

 # perf stat -e uncore_imc_1/cas_count_read/ -a --interval-print 10 -- sleep 1

 print interval < 100ms. The overhead percentage could be high in some
 cases. Please proceed with caution.
 #           time             counts unit events
      0.010200451               0.10 MiB  uncore_imc_1/cas_count_read/
      0.020475117               0.02 MiB  uncore_imc_1/cas_count_read/
      0.030692800               0.01 MiB  uncore_imc_1/cas_count_read/
      0.040948161               0.02 MiB  uncore_imc_1/cas_count_read/
      0.051159564               0.00 MiB  uncore_imc_1/cas_count_read/

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1443776674-42511-1-git-send-email-kan.liang@intel.com
[ Added warning about overhead when using sub 100ms intervals to the man page ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -128,8 +128,9 @@ perf stat --repeat 10 --null --sync --pre 'make -s O=defconfig-build/clean' -- m
 
 -I msecs::
 --interval-print msecs::
-	Print count deltas every N milliseconds (minimum: 100ms)
-	example: perf stat -I 1000 -e cycles -a sleep 5
+Print count deltas every N milliseconds (minimum: 10ms)
+The overhead percentage could be high in some cases, for instance with small, sub 100ms intervals.  Use with caution.
+	example: 'perf stat -I 1000 -e cycles -a sleep 5'
 
 --per-socket::
 Aggregate counts per processor socket for system-wide mode measurements.  This

tools/perf/builtin-stat.c

@@ -1179,7 +1179,7 @@ int cmd_stat(int argc, const char **argv, const char *prefix __maybe_unused)
 	OPT_STRING(0, "post", &post_cmd, "command",
 			"command to run after to the measured command"),
 	OPT_UINTEGER('I', "interval-print", &stat_config.interval,
-		    "print counts at regular interval in ms (>= 100)"),
+		    "print counts at regular interval in ms (>= 10)"),
 	OPT_SET_UINT(0, "per-socket", &stat_config.aggr_mode,
 		     "aggregate counts per processor socket", AGGR_SOCKET),
 	OPT_SET_UINT(0, "per-core", &stat_config.aggr_mode,
@@ -1332,9 +1332,14 @@ int cmd_stat(int argc, const char **argv, const char *prefix __maybe_unused)
 		thread_map__read_comms(evsel_list->threads);
 
 	if (interval && interval < 100) {
-		pr_err("print interval must be >= 100ms\n");
-		parse_options_usage(stat_usage, options, "I", 1);
-		goto out;
+		if (interval < 10) {
+			pr_err("print interval must be >= 10ms\n");
+			parse_options_usage(stat_usage, options, "I", 1);
+			goto out;
+		} else
+			pr_warning("print interval < 100ms. "
+				   "The overhead percentage could be high in some cases. "
+				   "Please proceed with caution.\n");
 	}
 
 	if (perf_evlist__alloc_stats(evsel_list, interval))