4.4-stable review patch. If anyone has any objections, please let me know.
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From: Steven Rostedt rostedt@goodmis.org
commit f73c52a5bcd1710994e53fbccc378c42b97a06b6 upstream.
Daniel Wagner reported a crash on the BeagleBone Black SoC.
This is a single CPU architecture, and does not have a functional arch_send_call_function_single_ipi() implementation which can crash the kernel if that is called.
As it only has one CPU, it shouldn't be called, but if the kernel is compiled for SMP, the push/pull RT scheduling logic now calls it for irq_work if the one CPU is overloaded, it can use that function to call itself and crash the kernel.
Ideally, we should disable the SCHED_FEAT(RT_PUSH_IPI) if the system only has a single CPU. But SCHED_FEAT is a constant if sched debugging is turned off. Another fix can also be used, and this should also help with normal SMP machines. That is, do not initiate the pull code if there's only one RT overloaded CPU, and that CPU happens to be the current CPU that is scheduling in a lower priority task.
Even on a system with many CPUs, if there's many RT tasks waiting to run on a single CPU, and that CPU schedules in another RT task of lower priority, it will initiate the PULL logic in case there's a higher priority RT task on another CPU that is waiting to run. But if there is no other CPU with waiting RT tasks, it will initiate the RT pull logic on itself (as it still has RT tasks waiting to run). This is a wasted effort.
Not only does this help with SMP code where the current CPU is the only one with RT overloaded tasks, it should also solve the issue that Daniel encountered, because it will prevent the PULL logic from executing, as there's only one CPU on the system, and the check added here will cause it to exit the RT pull code.
Reported-by: Daniel Wagner wagi@monom.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org Acked-by: Peter Zijlstra peterz@infradead.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Thomas Gleixner tglx@linutronix.de Cc: linux-rt-users linux-rt-users@vger.kernel.org Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") Link: http://lkml.kernel.org/r/20171202130454.4cbbfe8d@vmware.local.home Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/sched/rt.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
--- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1960,8 +1960,9 @@ static void pull_rt_task(struct rq *this bool resched = false; struct task_struct *p; struct rq *src_rq; + int rt_overload_count = rt_overloaded(this_rq);
- if (likely(!rt_overloaded(this_rq))) + if (likely(!rt_overload_count)) return;
/* @@ -1970,6 +1971,11 @@ static void pull_rt_task(struct rq *this */ smp_rmb();
+ /* If we are the only overloaded CPU do nothing */ + if (rt_overload_count == 1 && + cpumask_test_cpu(this_rq->cpu, this_rq->rd->rto_mask)) + return; + #ifdef HAVE_RT_PUSH_IPI if (sched_feat(RT_PUSH_IPI)) { tell_cpu_to_push(this_rq);