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SubjectRe: [PATCH 04/14] sched: SCHED_DEADLINE SMP-related data structures & logic.
On Tue, Oct 15, 2013 at 11:36:17AM +0200, Juri Lelli wrote:
> On 10/14/2013 02:03 PM, Peter Zijlstra wrote:
> > On Mon, Oct 14, 2013 at 12:43:36PM +0200, Juri Lelli wrote:
> >> +static inline void dl_set_overload(struct rq *rq)
> >> +{
> >> + if (!rq->online)
> >> + return;
> >> +
> >> + cpumask_set_cpu(rq->cpu, rq->rd->dlo_mask);
> >> + /*
> >> + * Must be visible before the overload count is
> >> + * set (as in sched_rt.c).
> >> + */
> >> + wmb();
> >> + atomic_inc(&rq->rd->dlo_count);
> >> +}
> >
> > Please, make that smp_wmb() and modify the comment to point to the
> > matching barrier ; I couldn't find one! Which suggests something is
> > amiss.
> >
> > Ideally we'd have something like smp_wmb__after_set_bit() but alas.
> >
>
> The only user of this function is pull_dl_task (that tries to pull only if at
> least one runqueue of the root_domain is overloaded). Surely makes sense to
> ensure that changes in the dlo_mask have to be visible before we check if we
> should look at that mask. Am I right if I say that the matching barrier is
> constituted by the spin_lock on this_rq acquired by schedule() before calling
> pre_schedule()?
>
> Same thing in rt_set_overload(), do we need to modify the comment also there?

So I haven't looked at the dl code, but for the RT code the below is
required.

Without that smp_rmb() in there we could actually miss seeing the
rto_mask bit.

---
kernel/sched/rt.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index e9304cdc26fe..a848f526b941 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -246,8 +246,10 @@ static inline void rt_set_overload(struct rq *rq)
* if we should look at the mask. It would be a shame
* if we looked at the mask, but the mask was not
* updated yet.
+ *
+ * Matched by the barrier in pull_rt_task().
*/
- wmb();
+ smp_wmb();
atomic_inc(&rq->rd->rto_count);
}

@@ -1626,6 +1628,12 @@ static int pull_rt_task(struct rq *this_rq)
if (likely(!rt_overloaded(this_rq)))
return 0;

+ /*
+ * Match the barrier from rt_set_overloaded; this guarantees that if we
+ * see overloaded we must also see the rto_mask bit.
+ */
+ smp_rmb();
+
for_each_cpu(cpu, this_rq->rd->rto_mask) {
if (this_cpu == cpu)
continue;

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