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authorPeter Zijlstra <peterz@infradead.org>2015-09-28 17:52:18 +0200
committerIngo Molnar <mingo@kernel.org>2015-10-06 17:08:14 +0200
commit609ca066386b2e64d4c0b0f55da327654962a0c9 (patch)
tree11b4faacc00bbc39d967374df633f6c352ce199b /kernel/sched
parentb99def8b961448f5b9a550dddeeb718e3975e7a6 (diff)
downloadop-kernel-dev-609ca066386b2e64d4c0b0f55da327654962a0c9.zip
op-kernel-dev-609ca066386b2e64d4c0b0f55da327654962a0c9.tar.gz
sched/core: Create preempt_count invariant
Assuming units of PREEMPT_DISABLE_OFFSET for preempt_count() numbers. Now that TASK_DEAD no longer results in preempt_count() == 3 during scheduling, we will always call context_switch() with preempt_count() == 2. However, we don't always end up with preempt_count() == 2 in finish_task_switch() because new tasks get created with preempt_count() == 1. Create FORK_PREEMPT_COUNT and set it to 2 and use that in the right places. Note that we cannot use INIT_PREEMPT_COUNT as that serves another purpose (boot). After this, preempt_count() is invariant across the context switch, with exception of PREEMPT_ACTIVE. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c23
1 files changed, 21 insertions, 2 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 530fe8ba..8d8722b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2504,6 +2504,18 @@ static struct rq *finish_task_switch(struct task_struct *prev)
struct mm_struct *mm = rq->prev_mm;
long prev_state;
+ /*
+ * The previous task will have left us with a preempt_count of 2
+ * because it left us after:
+ *
+ * schedule()
+ * preempt_disable(); // 1
+ * __schedule()
+ * raw_spin_lock_irq(&rq->lock) // 2
+ *
+ * Also, see FORK_PREEMPT_COUNT.
+ */
+
rq->prev_mm = NULL;
/*
@@ -2588,8 +2600,15 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
{
struct rq *rq;
- /* finish_task_switch() drops rq->lock and enables preemtion */
- preempt_disable();
+ /*
+ * New tasks start with FORK_PREEMPT_COUNT, see there and
+ * finish_task_switch() for details.
+ *
+ * finish_task_switch() will drop rq->lock() and lower preempt_count
+ * and the preempt_enable() will end up enabling preemption (on
+ * PREEMPT_COUNT kernels).
+ */
+
rq = finish_task_switch(prev);
balance_callback(rq);
preempt_enable();
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