linux-kernel - [PATCH 3.16.y-ckt 127/129] sched/autogroup: Fix failure to set cpu.rt_runtime

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Message-Id: <1432285114-9254-128-git-send-email-luis.henriques@canonical.com>
Date:	Fri, 22 May 2015 09:58:32 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Ingo Molnar <mingo@...nel.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 127/129] sched/autogroup: Fix failure to set cpu.rt_runtime_us

3.16.7-ckt12 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@...radead.org>

commit 1fe89e1b6d270aa0d3452c60d38461ea589594e3 upstream.

Because task_group() uses a cache of autogroup_task_group(), whose
output depends on sched_class, switching classes can generate
problems.

In particular, when started as fair, the cache points to the
autogroup, so when switching to RT the tg_rt_schedulable() test fails
for every cpu.rt_{runtime,period}_us change because now the autogroup
has tasks and no runtime.

Furthermore, going back to the previous semantics of varying
task_group() with sched_class has the down-side that the sched_debug
output varies as well, even though the task really is in the
autogroup.

Therefore add an autogroup exception to tg_has_rt_tasks() -- such that
both (all) task_group() usages in sched/core now have one. And remove
all the remnants of the variable task_group() output.

Reported-by: Zefan Li <lizefan@...wei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Stefan Bader <stefan.bader@...onical.com>
Fixes: 8323f26ce342 ("sched: Fix race in task_group()")
Link: http://lkml.kernel.org/r/20150209112237.GR5029@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 kernel/sched/auto_group.c | 6 +-----
 kernel/sched/core.c       | 6 ++++++
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index e73efba98301..798e912842d5 100644
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -87,8 +87,7 @@ static inline struct autogroup *autogroup_create(void)
 	 * so we don't have to move tasks around upon policy change,
 	 * or flail around trying to allocate bandwidth on the fly.
 	 * A bandwidth exception in __sched_setscheduler() allows
-	 * the policy change to proceed.  Thereafter, task_group()
-	 * returns &root_task_group, so zero bandwidth is required.
+	 * the policy change to proceed.
 	 */
 	free_rt_sched_group(tg);
 	tg->rt_se = root_task_group.rt_se;
@@ -115,9 +114,6 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
 	if (tg != &root_task_group)
 		return false;
 
-	if (p->sched_class != &fair_sched_class)
-		return false;
-
 	/*
 	 * We can only assume the task group can't go away on us if
 	 * autogroup_move_group() can see us on ->thread_group list.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 847922e4ba67..6356f644a47b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7348,6 +7348,12 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
 {
 	struct task_struct *g, *p;
 
+	/*
+	 * Autogroups do not have RT tasks; see autogroup_create().
+	 */
+	if (task_group_is_autogroup(tg))
+		return 0;
+
 	do_each_thread(g, p) {
 		if (rt_task(p) && task_rq(p)->rt.tg == tg)
 			return 1;
--
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