[PATCH 2/2] sched: Pull resched loop to __schedule() callers

From: Frederic Weisbecker
Date: Sun Dec 14 2014 - 21:25:04 EST

__schedule() disables preemption during its job and re-enables it
afterward without doing a preemption check to avoid recursion.

But if an event happens after the context switch which requires
rescheduling, we need to check again if a task of a higher priority
needs the CPU. A preempt irq can raise such a situation. To handle that,
__schedule() loops on need_resched().

But preempt_schedule_*() functions, which call __schedule(), also loop
on need_resched() to handle missed preempt irqs. Hence we end up with
the same loop happening twice.

Lets simplify that by attributing the need_resched() loop responsability
to all __schedule() callers.

There is a risk that the outer loop now handles reschedules that used
to be handled by the inner loop with the added overhead of caller details
(inc/dec of PREEMPT_ACTIVE, irq save/restore) but assuming those inner
rescheduling loop weren't too frequent, this shouldn't matter. Especially
since the whole preemption path is now loosing one loop in any case.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
kernel/sched/core.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 069a2d8..368c8f3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2748,6 +2748,10 @@ again:
* - explicit schedule() call
* - return from syscall or exception to user-space
* - return from interrupt-handler to user-space
+ *
+ * WARNING: all callers must re-check need_resched() afterward and reschedule
+ * accordingly in case an event triggered the need for rescheduling (such as
+ * an interrupt waking up a task) while preemption was disabled in __schedule().
static void __sched __schedule(void)
@@ -2756,7 +2760,6 @@ static void __sched __schedule(void)
struct rq *rq;
int cpu;

cpu = smp_processor_id();
rq = cpu_rq(cpu);
@@ -2821,8 +2824,6 @@ need_resched:

- if (need_resched())
- goto need_resched;

static inline void sched_submit_work(struct task_struct *tsk)
@@ -2842,7 +2843,9 @@ asmlinkage __visible void __sched schedule(void)
struct task_struct *tsk = current;

- __schedule();
+ do {
+ __schedule();
+ } while (need_resched());


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