lkml.org 
[lkml]   [2018]   [Sep]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[RFC 59/60] cosched: Handle non-atomicity during switches to and from coscheduling
Date
We cannot switch a task group from regular scheduling to coscheduling
atomically, as it would require locking the whole system. Instead,
the switch is done runqueue by runqueue via cosched_set_scheduled().

This means that other CPUs may see an intermediate state when locking
a bunch of runqueues, where the sdrq->is_root fields do not yield
a consistent picture across a task group.

Handle these cases.

Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
---
kernel/sched/fair.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 322a84ec9511..8da2033596ff 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -646,6 +646,15 @@ static struct cfs_rq *current_cfs(struct rq *rq)
{
struct sdrq *sdrq = READ_ONCE(rq->sdrq_data.current_sdrq);

+ /*
+ * We might race with concurrent is_root-changes, causing
+ * current_sdrq to reference an sdrq which is no longer
+ * !is_root. Counter that by ascending the tg-hierarchy
+ * until we find an sdrq with is_root.
+ */
+ while (sdrq->is_root && sdrq->tg_parent)
+ sdrq = sdrq->tg_parent;
+
return sdrq->cfs_rq;
}
#else
@@ -7141,6 +7150,23 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf

se = pick_next_entity(cfs_rq, curr);
cfs_rq = pick_next_cfs(se);
+
+#ifdef CONFIG_COSCHEDULING
+ if (cfs_rq && is_sd_se(se) && cfs_rq->sdrq.is_root) {
+ WARN_ON_ONCE(1); /* Untested code path */
+ /*
+ * Race with is_root update.
+ *
+ * We just moved downwards in the hierarchy via an
+ * SD-SE, the CFS-RQ should have is_root set to zero.
+ * However, a reconfiguration may be in progress. We
+ * basically ignore that reconfiguration.
+ *
+ * Contrary to the case below, there is nothing to fix
+ * as all the set_next_entity() calls are done later.
+ */
+ }
+#endif
} while (cfs_rq);

if (is_sd_se(se))
@@ -7192,6 +7218,48 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
se = pick_next_entity(cfs_rq, NULL);
set_next_entity(cfs_rq, se);
cfs_rq = pick_next_cfs(se);
+
+#ifdef CONFIG_COSCHEDULING
+ if (cfs_rq && is_sd_se(se) && cfs_rq->sdrq.is_root) {
+ /*
+ * Race with is_root update.
+ *
+ * We just moved downwards in the hierarchy via an
+ * SD-SE, the CFS-RQ should have is_root set to zero.
+ * However, a reconfiguration may be in progress. We
+ * basically ignore that reconfiguration, but we need
+ * to fix the picked path to correspond to that
+ * reconfiguration.
+ *
+ * Thus, we walk the hierarchy upwards again and do two
+ * things simultaneously:
+ *
+ * 1. put back picked entities which are not on the
+ * "correct" path,
+ * 2. pick the entities along the correct path.
+ *
+ * We do this until both paths upwards converge.
+ */
+ struct sched_entity *se2 = cfs_rq->sdrq.tg_se;
+ bool top = false;
+
+ WARN_ON_ONCE(1); /* Untested code path */
+ while (se && se != se2) {
+ if (!top) {
+ put_prev_entity(cfs_rq_of(se), se);
+ if (cfs_rq_of(se) == top_cfs_rq)
+ top = true;
+ }
+ if (top)
+ se = cfs_rq_of(se)->sdrq.tg_se;
+ else
+ se = parent_entity(se);
+ set_next_entity(cfs_rq_of(se2), se2);
+ se2 = parent_entity(se2);
+ }
+ }
+#endif
+
} while (cfs_rq);

retidle: __maybe_unused;
--
2.9.3.1.gcba166c.dirty
\
 
 \ /
  Last update: 2018-09-07 23:49    [W:0.806 / U:0.484 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site