[PATCH 3.2 053/221] sched/autogroup: Fix failure to set cpu.rt_runtime_us

Ben Hutchings
2015-05-05T01:56:28Z

From: Peter Zijlstra <peterz@infradead.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@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Stefan Bader <stefan.bader@canonical.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@kernel.org>
[bwh: Backported to 3.2: adjust filenames, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/sched_autogroup.c | 6 +-----
kernel/sched.c | 6 ++++++
2 files changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/sched_autogroup.c
+++ b/kernel/sched_autogroup.c
@@ -86,8 +86,7 @@ static inline struct autogroup *autogrou
* 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;
@@ -114,9 +113,6 @@ task_wants_autogroup(struct task_struct
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.
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9104,6 +9104,12 @@ static inline int tg_has_rt_tasks(struct
{
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) && rt_rq_of_se(&p->rt)->tg == tg)
return 1;