[PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[Michal Koutný]

Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
decayed into the load (leaf) list. We may ignore adding some ancestors
and therefore breaking tmp_alone_branch invariant. This broke LTP test
cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
("sched/fair: Ensure that the CFS parent is added after unthrottling").

I noticed the named test still fails even with the fix (but with low
probability, 1 in ~1000 executions of the test). The reason is when
bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.

Fix this by adding ancestors if we notice the unthrottled cfs_rq was
added to the load list.

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---

Changes since v2 [1]:
- jump to completion for loop, don't duplicate it
- singled out of the series (to handle a fix separately from the rest)

[1] https://lore.kernel.org/r/20210819175034.4577-2-mkoutny@suse.com/

 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff69f245b939..f6a05d9b5443 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	/* update hierarchical throttle state */
 	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
 
-	if (!cfs_rq->load.weight)
+	/* Nothing to run but something to decay (on_list)? Complete the branch */
+	if (!cfs_rq->load.weight) {
+		if (cfs_rq->on_list)
+			goto unthrottle_throttle;
 		return;
+	}
 
 	task_delta = cfs_rq->h_nr_running;
 	idle_task_delta = cfs_rq->idle_h_nr_running;
-- 
2.32.0

Re: [PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[Vincent Guittot]

On Fri, 17 Sept 2021 at 17:32, Michal Koutný <mkoutny@suse.com> wrote:
>
> Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
> list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
> decayed into the load (leaf) list. We may ignore adding some ancestors
> and therefore breaking tmp_alone_branch invariant. This broke LTP test
> cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
> ("sched/fair: Ensure that the CFS parent is added after unthrottling").
>
> I noticed the named test still fails even with the fix (but with low
> probability, 1 in ~1000 executions of the test). The reason is when
> bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
> the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.
>
> Fix this by adding ancestors if we notice the unthrottled cfs_rq was
> added to the load list.
>
> Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>
> Changes since v2 [1]:
> - jump to completion for loop, don't duplicate it
> - singled out of the series (to handle a fix separately from the rest)
>
> [1] https://lore.kernel.org/r/20210819175034.4577-2-mkoutny@suse.com/
>
>  kernel/sched/fair.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f245b939..f6a05d9b5443 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>         /* update hierarchical throttle state */
>         walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
>
> -       if (!cfs_rq->load.weight)
> +       /* Nothing to run but something to decay (on_list)? Complete the branch */
> +       if (!cfs_rq->load.weight) {
> +               if (cfs_rq->on_list)
> +                       goto unthrottle_throttle;
>                 return;
> +       }
>
>         task_delta = cfs_rq->h_nr_running;
>         idle_task_delta = cfs_rq->idle_h_nr_running;
> --
> 2.32.0
>

Re: [PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[Odin Ugedal]

fre. 17. sep. 2021 kl. 16:32 skrev Michal Koutný <mkoutny@suse.com>:
>
> Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
> list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
> decayed into the load (leaf) list. We may ignore adding some ancestors
> and therefore breaking tmp_alone_branch invariant. This broke LTP test
> cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
> ("sched/fair: Ensure that the CFS parent is added after unthrottling").
>
> I noticed the named test still fails even with the fix (but with low
> probability, 1 in ~1000 executions of the test). The reason is when
> bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
> the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.
>
> Fix this by adding ancestors if we notice the unthrottled cfs_rq was
> added to the load list.
>
> Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Thanks for catching this!

Reviewed-by: Odin Ugedal <odin@uged.al>

> Changes since v2 [1]:
> - jump to completion for loop, don't duplicate it
> - singled out of the series (to handle a fix separately from the rest)
>
> [1] https://lore.kernel.org/r/20210819175034.4577-2-mkoutny@suse.com/
>
>  kernel/sched/fair.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f245b939..f6a05d9b5443 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>         /* update hierarchical throttle state */
>         walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
>
> -       if (!cfs_rq->load.weight)
> +       /* Nothing to run but something to decay (on_list)? Complete the branch */
> +       if (!cfs_rq->load.weight) {
> +               if (cfs_rq->on_list)
> +                       goto unthrottle_throttle;
>                 return;
> +       }
>
>         task_delta = cfs_rq->h_nr_running;
>         idle_task_delta = cfs_rq->idle_h_nr_running;
> --
> 2.32.0
>

[tip: sched/urgent] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[tip-bot2 for Michal Koutný]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2630cde26711dab0d0b56a8be1616475be646d13
Gitweb:        https://git.kernel.org/tip/2630cde26711dab0d0b56a8be1616475be646d13
Author:        Michal Koutný <mkoutny@suse.com>
AuthorDate:    Fri, 17 Sep 2021 17:30:37 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 01 Oct 2021 13:57:57 +02:00

sched/fair: Add ancestors of unthrottled undecayed cfs_rq

Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
decayed into the load (leaf) list. We may ignore adding some ancestors
and therefore breaking tmp_alone_branch invariant. This broke LTP test
cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
("sched/fair: Ensure that the CFS parent is added after unthrottling").

I noticed the named test still fails even with the fix (but with low
probability, 1 in ~1000 executions of the test). The reason is when
bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.

Fix this by adding ancestors if we notice the unthrottled cfs_rq was
added to the load list.

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Odin Ugedal <odin@uged.al>
Link: https://lore.kernel.org/r/20210917153037.11176-1-mkoutny@suse.com
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff69f24..f6a05d9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	/* update hierarchical throttle state */
 	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
 
-	if (!cfs_rq->load.weight)
+	/* Nothing to run but something to decay (on_list)? Complete the branch */
+	if (!cfs_rq->load.weight) {
+		if (cfs_rq->on_list)
+			goto unthrottle_throttle;
 		return;
+	}
 
 	task_delta = cfs_rq->h_nr_running;
 	idle_task_delta = cfs_rq->idle_h_nr_running;