linux-kernel - [PATCH] perf/ftrace : Fix repetitious traces when specify a target task

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Date:   Tue, 10 Oct 2017 09:19:39 +0800
From:   Cheng Jian <cj.chengjian@...wei.com>
To:     <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
        <alexander.shishkin@...ux.intel.com>
CC:     <linux-kernel@...r.kernel.org>, <xiexiuqi@...wei.com>,
        <huawei.libin@...wei.com>, <cj.chengjian@...wei.com>
Subject: [PATCH] perf/ftrace : Fix repetitious traces when specify a target  task

When use perf to trace the sched_wakeup and sched_wakeup_new tracepoint,
there is a bug that output the same event repetitiously.
It can be reproduced by :

	perf record -e sched:sched_wakeup_new ./bug_fork

bug_fork is an demo that can generating wakeup_new events :

	the parent process does nothing but
	fork a child process, and then they both quit.

perf script :

	bug_fork  1078 [002]   184.669341: sched:sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000
        bug_fork  1078 [002]   184.670128: sched:sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000
	bug_fork  1078 [002]   184.670128: sched:sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000
	bug_fork  1078 [002]   184.670128: sched:sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000
	bug_fork  1078 [002]   184.670128: sched:sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000

but ftrace only show one event:

	bug_fork-1078  [002] d...   184.889159: sched_wakeup_new:
comm=bug_fork pid=1079 prio=120 target_cpu=000

perf script repeat prints wakeup_new events multiple times.

These events which trigger this issue not only monitor the current task,
but also specify a target task. For example, the sched_wakeup and
sched_wakeup_new tracepoint will be caught when the current task
wakeup the target task which we traced on.

commit e6dab5ffab59 ("perf/trace: Add ability to set a target task
for events") has designed a method to trace these events which
specify a target task. But there have tow issues when monitoring
multithreaded/multiprocess apps.

First, it match an event(such as wakeup/wakeup_new/stat_*) at the begin,
but the function doesn't return, the event will be matched again because
of task != current.

Second, due to these events are registered/mmaped at per-cpu
or per-thread(--per-thread), so perf_swevent_event will match
these events multiple times in the branch(task != current),
the number of repetitions is just the number of CPUs or threads.

perf_tp_event will only match an event event at a time,
so we will return after an event matched.

Signed-off-by: Cheng Jian <cj.chengjian@...wei.com>
---
 kernel/events/core.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index baa134c..5682ead 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7988,12 +7988,16 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,

 	/* Use the given event instead of the hlist */
 	if (event) {
-		if (perf_tp_event_match(event, &data, regs))
+		if (perf_tp_event_match(event, &data, regs)) {
 			perf_swevent_event(event, count, &data, regs);
+			goto out;
+		}
 	} else {
 		hlist_for_each_entry_rcu(event, head, hlist_entry) {
-			if (perf_tp_event_match(event, &data, regs))
+			if (perf_tp_event_match(event, &data, regs)) {
 				perf_swevent_event(event, count, &data, regs);
+				goto out;
+			}
 		}
 	}

@@ -8015,13 +8019,15 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
 				continue;
 			if (event->attr.config != entry->type)
 				continue;
-			if (perf_tp_event_match(event, &data, regs))
+			if (perf_tp_event_match(event, &data, regs)) {
 				perf_swevent_event(event, count, &data, regs);
+				break;
+			}
 		}
 unlock:
 		rcu_read_unlock();
 	}
-
+out:
 	perf_swevent_put_recursion_context(rctx);
 }
 EXPORT_SYMBOL_GPL(perf_tp_event);
-- 
1.8.3.1