Re: [PATCH] sched, autogroup: Fix failure when writing to cpu.rt_runtime_us

[Peter Zijlstra]

On Mon, Feb 09, 2015 at 12:22:37PM +0100, Peter Zijlstra wrote:
> Indeed, setting runtime=0 for the root group is a very bad thing
> regardless of this patch. It would disallow the kernel from creating RT
> threads, which it needs for 'correct' operation in a number of cases.
> 
> But lets make that a separate patch.

---
Subject: sched,rt: Avoid obvious configuration fail
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Mon Feb  9 12:23:20 CET 2015

Setting the root group's cpu.rt_runtime_us to 0 is a bad thing; it
would disallow the kernel creating RT tasks.

One can of course still set it to 1, which will (likely) still wreck
your kernel, but at least make it clear that setting it to 0 is not
good.

Collect both sanity checks into the one place while we're there.

Suggested-by: Zefan Li <lizefan@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7742,6 +7742,17 @@ static int tg_set_rt_bandwidth(struct ta
 {
 	int i, err = 0;
 
+	/*
+	 * Disallowing the root group RT runtime is BAD, it would disallow the
+	 * kernel creating (and or operating) RT threads.
+	 */
+	if (tg == &root_task_group && rt_runtime == 0)
+		return -EINVAL;
+
+	/* No period doesn't make any sense. */
+	if (rt_period == 0)
+		return -EINVAL;
+
 	mutex_lock(&rt_constraints_mutex);
 	read_lock(&tasklist_lock);
 	err = __rt_schedulable(tg, rt_period, rt_runtime);
@@ -7798,9 +7809,6 @@ static int sched_group_set_rt_period(str
 	rt_period = (u64)rt_period_us * NSEC_PER_USEC;
 	rt_runtime = tg->rt_bandwidth.rt_runtime;
 
-	if (rt_period == 0)
-		return -EINVAL;
-
 	return tg_set_rt_bandwidth(tg, rt_period, rt_runtime);
 }
 
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