From: Zhang Qiao <[email protected]>
When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
to the base level (around cfs_rq->min_vruntime), so that the entity
doesn't gain extra boost when placed backwards.
However, if the entity being placed wasn't executed for a long time, its
vruntime may get too far behind (e.g. while cfs_rq was executing a
low-weight hog), which can inverse the vruntime comparison due to s64
overflow. This results in the entity being placed with its original
vruntime way forwards, so that it will effectively never get to the cpu.
To prevent that, ignore the vruntime of the entity being placed if it
didn't execute for much longer than the characteristic sheduler time
scale.
Signed-off-by: Zhang Qiao <[email protected]>
[rkagan: formatted, adjusted commit log, comments, cutoff value]
Co-developed-by: Roman Kagan <[email protected]>
Signed-off-by: Roman Kagan <[email protected]>
---
v1 -> v2:
- add Zhang Qiao's s-o-b
- fix constant promotion on 32bit
kernel/sched/fair.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0f8736991427..717c3ca970e1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4656,6 +4656,7 @@ static void
place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
{
u64 vruntime = cfs_rq->min_vruntime;
+ u64 sleep_time;
/*
* The 'current' period is already promised to the current tasks,
@@ -4685,8 +4686,18 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
vruntime -= thresh;
}
- /* ensure we never gain time by being placed backwards. */
- se->vruntime = max_vruntime(se->vruntime, vruntime);
+ /*
+ * Pull vruntime of the entity being placed to the base level of
+ * cfs_rq, to prevent boosting it if placed backwards. If the entity
+ * slept for a long time, don't even try to compare its vruntime with
+ * the base as it may be too far off and the comparison may get
+ * inversed due to s64 overflow.
+ */
+ sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
+ if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
+ se->vruntime = vruntime;
+ else
+ se->vruntime = max_vruntime(se->vruntime, vruntime);
}
static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
--
2.34.1
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On Mon, Jan 30, 2023 at 01:22:16PM +0100, Roman Kagan wrote:
> From: Zhang Qiao <[email protected]>
>
> When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
> to the base level (around cfs_rq->min_vruntime), so that the entity
> doesn't gain extra boost when placed backwards.
>
> However, if the entity being placed wasn't executed for a long time, its
> vruntime may get too far behind (e.g. while cfs_rq was executing a
> low-weight hog), which can inverse the vruntime comparison due to s64
> overflow. This results in the entity being placed with its original
> vruntime way forwards, so that it will effectively never get to the cpu.
>
> To prevent that, ignore the vruntime of the entity being placed if it
> didn't execute for much longer than the characteristic sheduler time
> scale.
>
> Signed-off-by: Zhang Qiao <[email protected]>
> [rkagan: formatted, adjusted commit log, comments, cutoff value]
> Co-developed-by: Roman Kagan <[email protected]>
> Signed-off-by: Roman Kagan <[email protected]>
Thanks!
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 829c1651e9c4a6f78398d3e67651cef9bb6b42cc
Gitweb: https://git.kernel.org/tip/829c1651e9c4a6f78398d3e67651cef9bb6b42cc
Author: Zhang Qiao <[email protected]>
AuthorDate: Mon, 30 Jan 2023 13:22:16 +01:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Sat, 11 Feb 2023 11:18:09 +01:00
sched/fair: sanitize vruntime of entity being placed
When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
to the base level (around cfs_rq->min_vruntime), so that the entity
doesn't gain extra boost when placed backwards.
However, if the entity being placed wasn't executed for a long time, its
vruntime may get too far behind (e.g. while cfs_rq was executing a
low-weight hog), which can inverse the vruntime comparison due to s64
overflow. This results in the entity being placed with its original
vruntime way forwards, so that it will effectively never get to the cpu.
To prevent that, ignore the vruntime of the entity being placed if it
didn't execute for much longer than the characteristic sheduler time
scale.
[rkagan: formatted, adjusted commit log, comments, cutoff value]
Signed-off-by: Zhang Qiao <[email protected]>
Co-developed-by: Roman Kagan <[email protected]>
Signed-off-by: Roman Kagan <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
kernel/sched/fair.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c6c8e7f..ff4dbba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4652,6 +4652,7 @@ static void
place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
{
u64 vruntime = cfs_rq->min_vruntime;
+ u64 sleep_time;
/*
* The 'current' period is already promised to the current tasks,
@@ -4681,8 +4682,18 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
vruntime -= thresh;
}
- /* ensure we never gain time by being placed backwards. */
- se->vruntime = max_vruntime(se->vruntime, vruntime);
+ /*
+ * Pull vruntime of the entity being placed to the base level of
+ * cfs_rq, to prevent boosting it if placed backwards. If the entity
+ * slept for a long time, don't even try to compare its vruntime with
+ * the base as it may be too far off and the comparison may get
+ * inversed due to s64 overflow.
+ */
+ sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
+ if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
+ se->vruntime = vruntime;
+ else
+ se->vruntime = max_vruntime(se->vruntime, vruntime);
}
static void check_enqueue_throttle(struct cfs_rq *cfs_rq);