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sched/eevdf: Fix pick_eevdf() 

The old pick_eevdf() could fail to find the actual earliest eligible
deadline when it descended to the right looking for min_deadline, but
it turned out that that min_deadline wasn't actually eligible. In that
case we need to go back and search through any left branches we
skipped looking for the actual best _eligible_ min_deadline.

This is more expensive, but still O(log n), and at worst should only
involve descending two branches of the rbtree.

I've run this through a userspace stress test (thank you
tools/lib/rbtree.c), so hopefully this implementation doesn't miss any
corner cases.

Fixes: 147f3efaa2 ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/xm261qego72d.fsf_-_@google.com
This commit is contained in:
Benjamin Segall 2023-09-29 17:09:30 -07:00 committed by Peter Zijlstra
parent 8dafa9d0eb
commit b01db23d59
1 changed files with 58 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

72
kernel/sched/fair.c
View File

@ -872,14 +872,16 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
*
* Which allows an EDF like search on (sub)trees.
*/
static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
{
struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
struct sched_entity *curr = cfs_rq->curr;
struct sched_entity *best = NULL;
struct sched_entity *best_left = NULL;
if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
curr = NULL;
best = curr;
/*
* Once selected, run a task until it either becomes non-eligible or
@ -900,33 +902,75 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
}
/*
* If this entity has an earlier deadline than the previous
* best, take this one. If it also has the earliest deadline
* of its subtree, we're done.
* Now we heap search eligible trees for the best (min_)deadline
*/
if (!best || deadline_gt(deadline, best, se)) {
if (!best || deadline_gt(deadline, best, se))
best = se;
if (best->deadline == best->min_deadline)
/*
* Every se in a left branch is eligible, keep track of the
* branch with the best min_deadline
*/
if (node->rb_left) {
struct sched_entity *left = __node_2_se(node->rb_left);
if (!best_left || deadline_gt(min_deadline, best_left, left))
best_left = left;
/*
* min_deadline is in the left branch. rb_left and all
* descendants are eligible, so immediately switch to the second
* loop.
*/
if (left->min_deadline == se->min_deadline)
break;
}
/*
* If the earlest deadline in this subtree is in the fully
* eligible left half of our space, go there.
*/
/* min_deadline is at this node, no need to look right */
if (se->deadline == se->min_deadline)
break;
/* else min_deadline is in the right branch. */
node = node->rb_right;
}
/*
* We ran into an eligible node which is itself the best.
* (Or nr_running == 0 and both are NULL)
*/
if (!best_left || (s64)(best_left->min_deadline - best->deadline) > 0)
return best;
/*
* Now best_left and all of its children are eligible, and we are just
* looking for deadline == min_deadline
*/
node = &best_left->run_node;
while (node) {
struct sched_entity *se = __node_2_se(node);
/* min_deadline is the current node */
if (se->deadline == se->min_deadline)
return se;
/* min_deadline is in the left branch */
if (node->rb_left &&
__node_2_se(node->rb_left)->min_deadline == se->min_deadline) {
node = node->rb_left;
continue;
}
/* else min_deadline is in the right branch */
node = node->rb_right;
}
return NULL;
}
if (!best || (curr && deadline_gt(deadline, best, curr)))
best = curr;
static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
{
struct sched_entity *se = __pick_eevdf(cfs_rq);
if (unlikely(!best)) {
if (!se) {
struct sched_entity *left = __pick_first_entity(cfs_rq);
if (left) {
pr_err("EEVDF scheduling fail, picking leftmost\n");
@ -934,7 +978,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
}
}
return best;
return se;
}
#ifdef CONFIG_SCHED_DEBUG