[PATCH 3.19 172/177] sched/autogroup: Fix failure to set cpu.rt_runtime_us

From: Greg Kroah-Hartman
Date: Mon Mar 16 2015 - 11:22:28 EST

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


From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit 1fe89e1b6d270aa0d3452c60d38461ea589594e3 upstream.

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

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

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Fixes: 8323f26ce342 ("sched: Fix race in task_group()")
Link: http://lkml.kernel.org/r/20150209112237.GR5029@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/sched/auto_group.c | 6 +-----
kernel/sched/core.c | 6 ++++++
2 files changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -87,8 +87,7 @@ 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.
tg->rt_se = root_task_group.rt_se;
@@ -115,9 +114,6 @@ bool task_wants_autogroup(struct task_st
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/core.c
+++ b/kernel/sched/core.c
@@ -7618,6 +7618,12 @@ 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;
for_each_process_thread(g, p) {
if (rt_task(p) && task_group(p) == tg)
return 1;

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