On Mon, Sep 21, 2015 at 07:46:11PM +0200, Oleg Nesterov wrote:
> On 09/18, Peter Zijlstra wrote:
> >
> > the text is correct, right?
>
> Yes, it looks good to me and helpful.
>
> But damn. I forgot why exactly try_to_wake_up() needs rmb() after
> ->on_cpu check... It looks reasonable in any case, but I do not
> see any strong reason immediately.
I read it like the smp_rmb() we have for
acquire__after_spin_is_unlocked. Except, as you note below, we need to
need an smp_read_barrier_depends for control barriers as well....
(I'm starting to think we're having more control deps what we were
thinking...)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1947,7 +1947,13 @@ try_to_wake_up(struct task_struct *p, un
while (p->on_cpu)
cpu_relax();
/*
- * Pairs with the smp_wmb() in finish_lock_switch().
+ * Combined with the control dependency above, we have an effective
+ * smp_load_acquire() without the need for full barriers.
+ *
+ * Pairs with the smp_store_release() in finish_lock_switch().
+ *
+ * This ensures that tasks getting woken will be fully ordered against
+ * their previous state and preserve Program Order.
*/
smp_rmb();
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1073,6 +1073,9 @@ static inline void finish_lock_switch(st
* We must ensure this doesn't happen until the switch is completely
* finished.
*
+ * In particular, the load of prev->state in finish_task_switch() must
+ * happen before this.
+ *
* Pairs with the control dependency and rmb in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);
Updates the comments to clarify the release/acquire pair on p->on_cpu.
> Say,
>
> p->sched_contributes_to_load = !!task_contributes_to_load(p);
> p->state = TASK_WAKING;
>
> we can actually do this before "while (p->on_cpu)", afaics. However
> we must not do this before the previous p->on_rq check.
No, we must not touch the task before p->on_cpu is cleared, up until
that point the task is owned by the 'previous' CPU.
> So perhaps this rmb() helps to ensure task_contributes_to_load() can't
> happen before p->on_rq check...
>
> As for "p->state = TASK_WAKING" we have the control dependency in both
> cases. But the modern fashion suggests to use _CTRL().
Yes, but I'm not sure we should go write:
while (READ_ONCE_CTRL(p->on_cpu))
cpu_relax();
Or:
while (p->on_cpu)
cpu_relax();
smp_read_barrier_depends();
It seems to me that doing the smp_mb() (for Alpha) inside the loop might
be sub-optimal.
That said, it would be good if Paul (or anyone really) can explain to me
the reason for: 5af4692a75da ("smp: Make control dependencies work on
Alpha, improve documentation"). The Changelog simply states that Alpha
needs the mb, but not how/why etc.
> Although cpu_relax()
> should imply barrier(), but afaik this is not documented.
I think we're relying on that in many places..
> In short, I got lost ;) Now I don't even understand why we do not need
> another rmb() between p->on_rq and p->on_cpu. Suppose a thread T does
>
> set_current_state(...);
> schedule();
>
> it can be preempted in between, after that we have "on_rq && !on_cpu".
> Then it gets CPU again and calls schedule() which clears on_rq.
>
> What guarantees that if ttwu() sees on_rq == 0 cleared by schedule()
> then it can _not_ still see the old value of on_cpu == 0?
Right, let me go have a think about that ;-)
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