linux-kernel - Re: Testing tracer wakeup_rt: .. no entries found ..FAILED!

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Message-ID: <1343740419.27983.33.camel@gandalf.stny.rr.com>
Date:	Tue, 31 Jul 2012 09:13:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Steven Rostedt <srostedt@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: Testing tracer wakeup_rt: .. no entries found ..FAILED!

On Tue, 2012-07-31 at 15:50 +0300, Avi Kivity wrote:
> On 07/31/2012 03:43 PM, Steven Rostedt wrote:

> That would be better.  A hypervisor might be real-time capable (with
> some effort kvm can do this), so we don't want to turn off real time
> features just based on that.

It would only turn off if you enable selftests and the timing falied. If
the kvm had real time features, this most likely would fail anyway. But
that said, here's a patch that should solve this:

-- Steve


diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 1003a4d..2c00a69 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -1041,6 +1041,8 @@ static int trace_wakeup_test_thread(void *data)
 	set_current_state(TASK_INTERRUPTIBLE);
 	schedule();
 
+	complete(x);
+
 	/* we are awake, now wait to disappear */
 	while (!kthread_should_stop()) {
 		/*
@@ -1084,24 +1086,21 @@ trace_selftest_startup_wakeup(struct tracer *trace, struct trace_array *tr)
 	/* reset the max latency */
 	tracing_max_latency = 0;
 
-	/* sleep to let the RT thread sleep too */
-	msleep(100);
+	while (p->on_rq) {
+		/*
+		 * Sleep to make sure the RT thread is asleep too.
+		 * On virtual machines we can't rely on timings,
+		 * but we want to make sure this test still works.
+		 */
+		msleep(100);
+	}
 
-	/*
-	 * Yes this is slightly racy. It is possible that for some
-	 * strange reason that the RT thread we created, did not
-	 * call schedule for 100ms after doing the completion,
-	 * and we do a wakeup on a task that already is awake.
-	 * But that is extremely unlikely, and the worst thing that
-	 * happens in such a case, is that we disable tracing.
-	 * Honestly, if this race does happen something is horrible
-	 * wrong with the system.
-	 */
+	init_completion(&isrt);
 
 	wake_up_process(p);
 
-	/* give a little time to let the thread wake up */
-	msleep(100);
+	/* Wait for the task to wake up */
+	wait_for_completion(&isrt);
 
 	/* stop the tracing. */
 	tracing_stop();



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