[PATCH 4/5] perf top: switch to overwrite mode

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

perf_top__mmap_read has severe performance issue in
Knights Landing/Mill, when monitoring in heavy load system. It costs
several minutes to finish, which is unacceptable.

Currently, perf top is non overwrite mode. For non overwrite mode, it
tries to read everything in the ringbuffer and doesn't pause the
ringbuffer. Once there are lots of samples delivered persistently,
the processing time could be very long. Also, the latest samples could
be lost when the ringbuffer is full.

For overwrite mode, it takes a snapshot for the system by pausing the
ringbuffer, which could significantly reducing the processing time.
Also, the overwrite mode always keep the latest samples.
Considering the real time requirement for perf top, the overwrite mode
is more suitable for perf top.

Actually, perf top was overwrite mode. It is changed to non overwrite
mode since commit 93fc64f14472 ("perf top: Switch to non overwrite
mode"). It's better to change it back to overwrite mode.

There would be some records lost in overwrite mode because of pausing
the ringbuffer. It has little impact for the accuracy of the snapshot
and could be tolerant.
The lost events checking is removed.

Unconditionally wait 100 ms before each snapshot. It also reduce the
overhead caused by pausing ringbuffer, especially on light load system.

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 tools/perf/builtin-top.c       | 30 ++++++++++++++----------------
 tools/perf/ui/browsers/hists.c | 12 +++++++++---
 2 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 540461f..721d786 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -283,16 +283,6 @@ static void perf_top__print_sym_table(struct perf_top *top)
 
 	printf("%-*.*s\n", win_width, win_width, graph_dotted_line);
 
-	if (hists->stats.nr_lost_warned !=
-	    hists->stats.nr_events[PERF_RECORD_LOST]) {
-		hists->stats.nr_lost_warned =
-			      hists->stats.nr_events[PERF_RECORD_LOST];
-		color_fprintf(stdout, PERF_COLOR_RED,
-			      "WARNING: LOST %d chunks, Check IO/CPU overload",
-			      hists->stats.nr_lost_warned);
-		++printed;
-	}
-
 	if (top->sym_filter_entry) {
 		perf_top__show_details(top);
 		return;
@@ -807,14 +797,19 @@ static void perf_event__process_sample(struct perf_tool *tool,
 
 static void perf_top__mmap_read_idx(struct perf_top *top, int idx)
 {
+	struct perf_mmap *md = &top->evlist->overwrite_mmap[idx];
 	struct perf_sample sample;
 	struct perf_evsel *evsel;
 	struct perf_session *session = top->session;
 	union perf_event *event;
 	struct machine *machine;
+	unsigned long size;
+	u64 end, start;
 	int ret;
 
-	while ((event = perf_evlist__mmap_read(top->evlist, idx)) != NULL) {
+	perf_mmap__read_catchup(md, true, &start, &end, &size);
+
+	while ((event = perf_mmap__read_backward(md, &start, end)) != NULL) {
 		ret = perf_evlist__parse_sample(top->evlist, event, &sample);
 		if (ret) {
 			pr_err("Can't parse sample, err = %d\n", ret);
@@ -869,16 +864,21 @@ static void perf_top__mmap_read_idx(struct perf_top *top, int idx)
 		} else
 			++session->evlist->stats.nr_unknown_events;
 next_event:
-		perf_evlist__mmap_consume(top->evlist, idx);
+		perf_mmap__consume(md, true);
 	}
+
+	perf_mmap__read_done(md);
 }
 
 static void perf_top__mmap_read(struct perf_top *top)
 {
 	int i;
 
+	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_DATA_PENDING);
 	for (i = 0; i < top->evlist->nr_mmaps; i++)
 		perf_top__mmap_read_idx(top, i);
+	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_EMPTY);
+	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_RUNNING);
 }
 
 static int perf_top__start_counters(struct perf_top *top)
@@ -1029,12 +1029,9 @@ static int __cmd_top(struct perf_top *top)
 	}
 
 	while (!done) {
-		u64 hits = top->samples;
-
 		perf_top__mmap_read(top);
 
-		if (hits == top->samples)
-			ret = perf_evlist__poll(top->evlist, 100);
+		ret = perf_evlist__poll(top->evlist, 100);
 
 		if (resize) {
 			perf_top__resize(top);
@@ -1127,6 +1124,7 @@ int cmd_top(int argc, const char **argv)
 				.uses_mmap   = true,
 			},
 			.proc_map_timeout    = 500,
+			.overwrite	= 1,
 		},
 		.max_stack	     = sysctl_perf_event_max_stack,
 		.sym_pcnt_filter     = 5,
diff --git a/tools/perf/ui/browsers/hists.c b/tools/perf/ui/browsers/hists.c
index 68146f4..56023e4 100644
--- a/tools/perf/ui/browsers/hists.c
+++ b/tools/perf/ui/browsers/hists.c
@@ -638,8 +638,13 @@ int hist_browser__run(struct hist_browser *browser, const char *help)
 			nr_entries = hist_browser__nr_entries(browser);
 			ui_browser__update_nr_entries(&browser->b, nr_entries);
 
-			if (browser->hists->stats.nr_lost_warned !=
-			    browser->hists->stats.nr_events[PERF_RECORD_LOST]) {
+			/*
+			 * Don't print lost events warning for perf top,
+			 * because it is overwrite mode.
+			 * Perf top is the only tool which has hbt timer.
+			 */
+			if ((browser->hists->stats.nr_lost_warned !=
+			    browser->hists->stats.nr_events[PERF_RECORD_LOST]) && !hbt) {
 				browser->hists->stats.nr_lost_warned =
 					browser->hists->stats.nr_events[PERF_RECORD_LOST];
 				ui_browser__warn_lost_events(&browser->b);
@@ -3203,7 +3208,8 @@ static int perf_evsel_menu__run(struct perf_evsel_menu *menu,
 		case K_TIMER:
 			hbt->timer(hbt->arg);
 
-			if (!menu->lost_events_warned && menu->lost_events) {
+			if (!menu->lost_events_warned &&
+			    menu->lost_events && !hbt) {
 				ui_browser__warn_lost_events(&menu->b);
 				menu->lost_events_warned = true;
 			}
-- 
2.5.5