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author | Peter Zijlstra <peterz@infradead.org> | 2015-09-28 17:52:18 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2015-10-06 17:08:14 +0200 |
commit | 609ca066386b2e64d4c0b0f55da327654962a0c9 (patch) | |
tree | 11b4faacc00bbc39d967374df633f6c352ce199b /kernel/sched | |
parent | b99def8b961448f5b9a550dddeeb718e3975e7a6 (diff) | |
download | linux-609ca066386b2e64d4c0b0f55da327654962a0c9.tar.gz linux-609ca066386b2e64d4c0b0f55da327654962a0c9.tar.bz2 linux-609ca066386b2e64d4c0b0f55da327654962a0c9.zip |
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.c | 23 |
1 files changed, 21 insertions, 2 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 530fe8baa645..8d8722b84dee 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(); |