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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/