linux-kernel - [PATCH v2] perf stat: avoid 10ms limit for printing event counts

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Date:   Wed, 28 Mar 2018 08:11:15 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] perf stat: avoid 10ms limit for printing event counts

Currently print count interval for performance counters values is 
limited by 10ms so reading the values at frequencies higher than 100Hz 
is restricted by the tool.

This change avoids that limitation and makes perf stat -I possible 
on frequencies up to 1KHz and, to some extent, makes perf stat -I 
to be on-par with perf record sampling profiling.

When running perf stat -I for monitoring e.g. PCIe uncore counters and 
at the same time profiling some I/O workload by perf record e.g. for 
cpu-cycles and context switches, it is then possible to build and 
observe good-enough consolidated CPU/OS/IO(Uncore) performance picture 
for that workload.

Printing the warning doesn't make sense in case you have output to 
the console because you quickly get your screen scrolled down. 
If the interval is small you may even skip the warning regardless 
of -v option.

Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
---

Distinction from v1 is that the warning message regarding possible 
overhead is moved from code to help text generated by per stat -h.

 tools/perf/builtin-stat.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index f5c454855908..147a27e8c937 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -1943,7 +1943,8 @@ static const struct option stat_options[] = {
 	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 (>= 10)"),
+		    "print counts at regular interval in ms "
+		    "(overhead is possible for values <= 100ms)"),
 	OPT_INTEGER(0, "interval-count", &stat_config.times,
 		    "print counts for fixed number of times"),
 	OPT_UINTEGER(0, "timeout", &stat_config.timeout,
@@ -2923,17 +2924,6 @@ int cmd_stat(int argc, const char **argv)
 		}
 	}

-	if (interval && interval < 100) {
-		if (interval < 10) {
-			pr_err("print interval must be >= 10ms\n");
-			parse_options_usage(stat_usage, stat_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 (stat_config.times && interval)
 		interval_count = true;
 	else if (stat_config.times && !interval) {