On Thu, Jul 08, 2021 at 09:54:06AM +0200 Peter Zijlstra wrote:
On Thu, Jul 08, 2021 at 09:48:03AM +0200, Peter Zijlstra wrote:
On Thu, Jul 08, 2021 at 09:26:26AM +0200, Peter Zijlstra wrote:
On Wed, Jul 07, 2021 at 03:04:57PM -0400, Phil Auld wrote:
On systems with weaker memory ordering (e.g. power) commit dbfb089d360b ("sched: Fix loadavg accounting race") causes increasing values of load average (via rq->calc_load_active and calc_load_tasks) due to the wakeup CPU not always seeing the write to task->sched_contributes_to_load in __schedule(). Missing that we fail to decrement nr_uninterruptible when waking up a task which incremented nr_uninterruptible when it slept.
The rq->lock serialization is insufficient across different rq->locks.
Add smp_wmb() to schedule and smp_rmb() before the read in ttwu_do_activate().
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4ca80df205ce..ced7074716eb 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2992,6 +2992,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags, lockdep_assert_held(&rq->lock);
- /* Pairs with smp_wmb in __schedule() */
- smp_rmb(); if (p->sched_contributes_to_load) rq->nr_uninterruptible--;
Is this really needed ?! (this question is a big fat clue the comment is insufficient). AFAICT try_to_wake_up() has a LOAD-ACQUIRE on p->on_rq and hence the p->sched_contributed_to_load must already happen after.
@@ -5084,6 +5086,11 @@ static void __sched notrace __schedule(bool preempt) !(prev_state & TASK_NOLOAD) && !(prev->flags & PF_FROZEN);
/** Make sure the previous write is ordered before p->on_rq etc so* that it is visible to other cpus in the wakeup path (ttwu_do_activate()).*/smp_wmb(); if (prev->sched_contributes_to_load) rq->nr_uninterruptible++;That comment is terrible, look at all the other barrier comments around there for clues; in effect you're worrying about:
p->sched_contributes_to_load = X R1 = p->on_rq WMB RMB p->on_rq = Y R2 = p->sched_contributes_to_load
Right?
Bah bah bah.. I so detest having to add barriers here for silly accounting. Let me think about this a little.
I got the below:
__schedule() ttwu()
rq_lock() raw_spin_lock(&p->pi_lock) smp_mb__after_spinlock(); smp_mb__after_spinlock();
p->sched_contributes_to_load = X; if (READ_ONCE(p->on_rq) && ...) goto unlock; smp_acquire__after_ctrl_dep();
smp_cond_load_acquire(&p->on_cpu, !VAL)deactivate_task() p->on_rq = 0;
context_switch() finish_task_switch() finish_task() smp_store_release(p->on_cpu, 0);
ttwu_queue() rq_lock() ttwu_do_activate() if (p->sched_contributes_to_load) ... rq_unlock() raw_spin_unlock(&p->pi_lock); finish_lock_switch() rq_unlock();The only way for ttwu() to end up in an enqueue, is if it did a LOAD-ACQUIRE on ->on_cpu,
That's not completely true; there's the WF_ON_CPU case, but in that scenario we IPI the CPU doing __schedule and it becomes simple UP/PO and everything must trivially work.
but that orders with the STORE-RELEASE on the same, which ensures the p->sched_contributes_to_load LOAD must happen after the STORE.
What am I missing? Your Changelog/comments provide insufficient clues..
Sorry... I don't have a nice diagram. I'm still looking at what all those macros actually mean on the various architectures.
"Works great in practice but how does it work in theory?" :)
Using what you have above I get the same thing. It looks like it should be ordered but in practice it's not, and ordering it "more" as I did in the patch, fixes it.
Is it possible that the bit field is causing some of the assumptions about ordering in those various macros to be off?
I notice in all the comments about smp_mb__after_spinlock etc, it's always WRITE_ONCE/READ_ONCE on the variables in question but we can't do that with the bit field.
I appreciate your time on this.
Cheers, Phil