It is easy for a cfs_rq to become throttled even when it has no enqueued
entities (for example, if we have just put_prev()'d the last runnable
task of the cfs_rq, and the cfs_rq is out of quota).
Avoid accounting this time towards total throttle time, since it
otherwise falsely inflates the stats.
Note that the dequeue path is special, since we normally disallow
migrations when a task is in a throttled hierarchy (see
throttled_lb_pair()).
Signed-off-by: Josh Don <[email protected]>
---
v3: fix bug with nested limits
kernel/sched/fair.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6189d1a45635..72571d48559a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4787,6 +4787,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
}
static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
+static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq);
static inline bool cfs_bandwidth_used(void);
@@ -4873,8 +4874,14 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if (cfs_rq->nr_running == 1) {
check_enqueue_throttle(cfs_rq);
- if (!throttled_hierarchy(cfs_rq))
+ if (!throttled_hierarchy(cfs_rq)) {
list_add_leaf_cfs_rq(cfs_rq);
+ } else {
+#ifdef CONFIG_CFS_BANDWIDTH
+ if (cfs_rq_throttled(cfs_rq) && !cfs_rq->throttled_clock)
+ cfs_rq->throttled_clock = rq_clock(rq_of(cfs_rq));
+#endif
+ }
}
}
@@ -5480,7 +5487,9 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq)
* throttled-list. rq->lock protects completion.
*/
cfs_rq->throttled = 1;
- cfs_rq->throttled_clock = rq_clock(rq);
+ SCHED_WARN_ON(cfs_rq->throttled_clock);
+ if (cfs_rq->nr_running)
+ cfs_rq->throttled_clock = rq_clock(rq);
return true;
}
@@ -5498,7 +5507,10 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
update_rq_clock(rq);
raw_spin_lock(&cfs_b->lock);
- cfs_b->throttled_time += rq_clock(rq) - cfs_rq->throttled_clock;
+ if (cfs_rq->throttled_clock) {
+ cfs_b->throttled_time += rq_clock(rq) - cfs_rq->throttled_clock;
+ cfs_rq->throttled_clock = 0;
+ }
list_del_rcu(&cfs_rq->throttled_list);
raw_spin_unlock(&cfs_b->lock);
--
2.41.0.162.gfafddb0af9-goog
We currently export the total throttled time for cgroups that are given
a bandwidth limit. This patch extends this accounting to also account
the total time that each children cgroup has been throttled.
This is useful to understand the degree to which children have been
affected by the throttling control. Children which are not runnable
during the entire throttled period, for example, will not show any
self-throttling time during this period.
Expose this in a new interface, 'cpu.stat.local', which is similar to
how non-hierarchical events are accounted in 'memory.events.local'.
Signed-off-by: Josh Don <[email protected]>
---
v3: fix bug with nested limits
v2:
- moved export to new cpu.stat.local file, per Tejun's recommendation
include/linux/cgroup-defs.h | 2 ++
kernel/cgroup/cgroup.c | 34 ++++++++++++++++++++++++++++
kernel/sched/core.c | 44 +++++++++++++++++++++++++++++++++++++
kernel/sched/fair.c | 21 +++++++++++++++++-
kernel/sched/sched.h | 2 ++
5 files changed, 102 insertions(+), 1 deletion(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 8a0d5466c7be..ae20dbb885d6 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -661,6 +661,8 @@ struct cgroup_subsys {
void (*css_rstat_flush)(struct cgroup_subsys_state *css, int cpu);
int (*css_extra_stat_show)(struct seq_file *seq,
struct cgroup_subsys_state *css);
+ int (*css_local_stat_show)(struct seq_file *seq,
+ struct cgroup_subsys_state *css);
int (*can_attach)(struct cgroup_taskset *tset);
void (*cancel_attach)(struct cgroup_taskset *tset);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 2d3d13e52333..bfe063225d3b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3726,6 +3726,36 @@ static int cpu_stat_show(struct seq_file *seq, void *v)
return ret;
}
+static int __maybe_unused cgroup_local_stat_show(struct seq_file *seq,
+ struct cgroup *cgrp, int ssid)
+{
+ struct cgroup_subsys *ss = cgroup_subsys[ssid];
+ struct cgroup_subsys_state *css;
+ int ret;
+
+ if (!ss->css_local_stat_show)
+ return 0;
+
+ css = cgroup_tryget_css(cgrp, ss);
+ if (!css)
+ return 0;
+
+ ret = ss->css_local_stat_show(seq, css);
+ css_put(css);
+ return ret;
+}
+
+static int cpu_local_stat_show(struct seq_file *seq, void *v)
+{
+ struct cgroup __maybe_unused *cgrp = seq_css(seq)->cgroup;
+ int ret = 0;
+
+#ifdef CONFIG_CGROUP_SCHED
+ ret = cgroup_local_stat_show(seq, cgrp, cpu_cgrp_id);
+#endif
+ return ret;
+}
+
#ifdef CONFIG_PSI
static int cgroup_io_pressure_show(struct seq_file *seq, void *v)
{
@@ -5276,6 +5306,10 @@ static struct cftype cgroup_base_files[] = {
.name = "cpu.stat",
.seq_show = cpu_stat_show,
},
+ {
+ .name = "cpu.stat.local",
+ .seq_show = cpu_local_stat_show,
+ },
{ } /* terminate */
};
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d9a04faa48a1..0de483190a57 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11140,6 +11140,27 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
return 0;
}
+
+static u64 throttled_time_self(struct task_group *tg)
+{
+ int i;
+ u64 total = 0;
+
+ for_each_possible_cpu(i) {
+ total += READ_ONCE(tg->cfs_rq[i]->throttled_clock_self_time);
+ }
+
+ return total;
+}
+
+static int cpu_cfs_local_stat_show(struct seq_file *sf, void *v)
+{
+ struct task_group *tg = css_tg(seq_css(sf));
+
+ seq_printf(sf, "throttled_time %llu\n", throttled_time_self(tg));
+
+ return 0;
+}
#endif /* CONFIG_CFS_BANDWIDTH */
#endif /* CONFIG_FAIR_GROUP_SCHED */
@@ -11216,6 +11237,10 @@ static struct cftype cpu_legacy_files[] = {
.name = "stat",
.seq_show = cpu_cfs_stat_show,
},
+ {
+ .name = "stat.local",
+ .seq_show = cpu_cfs_local_stat_show,
+ },
#endif
#ifdef CONFIG_RT_GROUP_SCHED
{
@@ -11272,6 +11297,24 @@ static int cpu_extra_stat_show(struct seq_file *sf,
return 0;
}
+static int cpu_local_stat_show(struct seq_file *sf,
+ struct cgroup_subsys_state *css)
+{
+#ifdef CONFIG_CFS_BANDWIDTH
+ {
+ struct task_group *tg = css_tg(css);
+ u64 throttled_self_usec;
+
+ throttled_self_usec = throttled_time_self(tg);
+ do_div(throttled_self_usec, NSEC_PER_USEC);
+
+ seq_printf(sf, "throttled_usec %llu\n",
+ throttled_self_usec);
+ }
+#endif
+ return 0;
+}
+
#ifdef CONFIG_FAIR_GROUP_SCHED
static u64 cpu_weight_read_u64(struct cgroup_subsys_state *css,
struct cftype *cft)
@@ -11450,6 +11493,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
.css_released = cpu_cgroup_css_released,
.css_free = cpu_cgroup_css_free,
.css_extra_stat_show = cpu_extra_stat_show,
+ .css_local_stat_show = cpu_local_stat_show,
#ifdef CONFIG_RT_GROUP_SCHED
.can_attach = cpu_cgroup_can_attach,
#endif
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 72571d48559a..b1bccef78b90 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4878,8 +4878,12 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
list_add_leaf_cfs_rq(cfs_rq);
} else {
#ifdef CONFIG_CFS_BANDWIDTH
+ struct rq *rq = rq_of(cfs_rq);
+
if (cfs_rq_throttled(cfs_rq) && !cfs_rq->throttled_clock)
- cfs_rq->throttled_clock = rq_clock(rq_of(cfs_rq));
+ cfs_rq->throttled_clock = rq_clock(rq);
+ if (!cfs_rq->throttled_clock_self)
+ cfs_rq->throttled_clock_self = rq_clock(rq);
#endif
}
}
@@ -5384,6 +5388,17 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
/* Add cfs_rq with load or one or more already running entities to the list */
if (!cfs_rq_is_decayed(cfs_rq))
list_add_leaf_cfs_rq(cfs_rq);
+
+ if (cfs_rq->throttled_clock_self) {
+ u64 delta = rq_clock(rq) - cfs_rq->throttled_clock_self;
+
+ cfs_rq->throttled_clock_self = 0;
+
+ if (SCHED_WARN_ON((s64)delta < 0))
+ delta = 0;
+
+ cfs_rq->throttled_clock_self_time += delta;
+ }
}
return 0;
@@ -5398,6 +5413,10 @@ static int tg_throttle_down(struct task_group *tg, void *data)
if (!cfs_rq->throttle_count) {
cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
list_del_leaf_cfs_rq(cfs_rq);
+
+ SCHED_WARN_ON(cfs_rq->throttled_clock_self);
+ if (cfs_rq->nr_running)
+ cfs_rq->throttled_clock_self = rq_clock(rq);
}
cfs_rq->throttle_count++;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 556496c77dc2..46f8a01ff405 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -636,6 +636,8 @@ struct cfs_rq {
u64 throttled_clock;
u64 throttled_clock_pelt;
u64 throttled_clock_pelt_time;
+ u64 throttled_clock_self;
+ u64 throttled_clock_self_time;
int throttled;
int throttle_count;
struct list_head throttled_list;
--
2.41.0.162.gfafddb0af9-goog
On Tue, Jun 20, 2023 at 11:32:47AM -0700, Josh Don wrote:
> We currently export the total throttled time for cgroups that are given
> a bandwidth limit. This patch extends this accounting to also account
> the total time that each children cgroup has been throttled.
>
> This is useful to understand the degree to which children have been
> affected by the throttling control. Children which are not runnable
> during the entire throttled period, for example, will not show any
> self-throttling time during this period.
>
> Expose this in a new interface, 'cpu.stat.local', which is similar to
> how non-hierarchical events are accounted in 'memory.events.local'.
>
> Signed-off-by: Josh Don <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Thanks.
--
tejun
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 79462e8c879afc7895b30014d31e2c1fd629bb1f
Gitweb: https://git.kernel.org/tip/79462e8c879afc7895b30014d31e2c1fd629bb1f
Author: Josh Don <[email protected]>
AuthorDate: Tue, 20 Jun 2023 11:32:46 -07:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 13 Jul 2023 15:21:49 +02:00
sched: don't account throttle time for empty groups
It is easy for a cfs_rq to become throttled even when it has no enqueued
entities (for example, if we have just put_prev()'d the last runnable
task of the cfs_rq, and the cfs_rq is out of quota).
Avoid accounting this time towards total throttle time, since it
otherwise falsely inflates the stats.
Note that the dequeue path is special, since we normally disallow
migrations when a task is in a throttled hierarchy (see
throttled_lb_pair()).
Signed-off-by: Josh Don <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
kernel/sched/fair.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a80a739..51ccae7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4787,6 +4787,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
}
static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
+static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq);
static inline bool cfs_bandwidth_used(void);
@@ -4873,8 +4874,14 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if (cfs_rq->nr_running == 1) {
check_enqueue_throttle(cfs_rq);
- if (!throttled_hierarchy(cfs_rq))
+ if (!throttled_hierarchy(cfs_rq)) {
list_add_leaf_cfs_rq(cfs_rq);
+ } else {
+#ifdef CONFIG_CFS_BANDWIDTH
+ if (cfs_rq_throttled(cfs_rq) && !cfs_rq->throttled_clock)
+ cfs_rq->throttled_clock = rq_clock(rq_of(cfs_rq));
+#endif
+ }
}
}
@@ -5480,7 +5487,9 @@ done:
* throttled-list. rq->lock protects completion.
*/
cfs_rq->throttled = 1;
- cfs_rq->throttled_clock = rq_clock(rq);
+ SCHED_WARN_ON(cfs_rq->throttled_clock);
+ if (cfs_rq->nr_running)
+ cfs_rq->throttled_clock = rq_clock(rq);
return true;
}
@@ -5498,7 +5507,10 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
update_rq_clock(rq);
raw_spin_lock(&cfs_b->lock);
- cfs_b->throttled_time += rq_clock(rq) - cfs_rq->throttled_clock;
+ if (cfs_rq->throttled_clock) {
+ cfs_b->throttled_time += rq_clock(rq) - cfs_rq->throttled_clock;
+ cfs_rq->throttled_clock = 0;
+ }
list_del_rcu(&cfs_rq->throttled_list);
raw_spin_unlock(&cfs_b->lock);
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 677ea015f231aa38b3972aa7be54ecd2637e99fd
Gitweb: https://git.kernel.org/tip/677ea015f231aa38b3972aa7be54ecd2637e99fd
Author: Josh Don <[email protected]>
AuthorDate: Tue, 20 Jun 2023 11:32:47 -07:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 13 Jul 2023 15:21:49 +02:00
sched: add throttled time stat for throttled children
We currently export the total throttled time for cgroups that are given
a bandwidth limit. This patch extends this accounting to also account
the total time that each children cgroup has been throttled.
This is useful to understand the degree to which children have been
affected by the throttling control. Children which are not runnable
during the entire throttled period, for example, will not show any
self-throttling time during this period.
Expose this in a new interface, 'cpu.stat.local', which is similar to
how non-hierarchical events are accounted in 'memory.events.local'.
Signed-off-by: Josh Don <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
include/linux/cgroup-defs.h | 2 ++-
kernel/cgroup/cgroup.c | 34 ++++++++++++++++++++++++++++-
kernel/sched/core.c | 44 ++++++++++++++++++++++++++++++++++++-
kernel/sched/fair.c | 21 ++++++++++++++++-
kernel/sched/sched.h | 2 ++-
5 files changed, 102 insertions(+), 1 deletion(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 8a0d546..ae20dbb 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -661,6 +661,8 @@ struct cgroup_subsys {
void (*css_rstat_flush)(struct cgroup_subsys_state *css, int cpu);
int (*css_extra_stat_show)(struct seq_file *seq,
struct cgroup_subsys_state *css);
+ int (*css_local_stat_show)(struct seq_file *seq,
+ struct cgroup_subsys_state *css);
int (*can_attach)(struct cgroup_taskset *tset);
void (*cancel_attach)(struct cgroup_taskset *tset);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index bfe3cd8..4e3ee13 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3685,6 +3685,36 @@ static int cpu_stat_show(struct seq_file *seq, void *v)
return ret;
}
+static int __maybe_unused cgroup_local_stat_show(struct seq_file *seq,
+ struct cgroup *cgrp, int ssid)
+{
+ struct cgroup_subsys *ss = cgroup_subsys[ssid];
+ struct cgroup_subsys_state *css;
+ int ret;
+
+ if (!ss->css_local_stat_show)
+ return 0;
+
+ css = cgroup_tryget_css(cgrp, ss);
+ if (!css)
+ return 0;
+
+ ret = ss->css_local_stat_show(seq, css);
+ css_put(css);
+ return ret;
+}
+
+static int cpu_local_stat_show(struct seq_file *seq, void *v)
+{
+ struct cgroup __maybe_unused *cgrp = seq_css(seq)->cgroup;
+ int ret = 0;
+
+#ifdef CONFIG_CGROUP_SCHED
+ ret = cgroup_local_stat_show(seq, cgrp, cpu_cgrp_id);
+#endif
+ return ret;
+}
+
#ifdef CONFIG_PSI
static int cgroup_io_pressure_show(struct seq_file *seq, void *v)
{
@@ -5235,6 +5265,10 @@ static struct cftype cgroup_base_files[] = {
.name = "cpu.stat",
.seq_show = cpu_stat_show,
},
+ {
+ .name = "cpu.stat.local",
+ .seq_show = cpu_local_stat_show,
+ },
{ } /* terminate */
};
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c52c2eb..2291f9d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11139,6 +11139,27 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
return 0;
}
+
+static u64 throttled_time_self(struct task_group *tg)
+{
+ int i;
+ u64 total = 0;
+
+ for_each_possible_cpu(i) {
+ total += READ_ONCE(tg->cfs_rq[i]->throttled_clock_self_time);
+ }
+
+ return total;
+}
+
+static int cpu_cfs_local_stat_show(struct seq_file *sf, void *v)
+{
+ struct task_group *tg = css_tg(seq_css(sf));
+
+ seq_printf(sf, "throttled_time %llu\n", throttled_time_self(tg));
+
+ return 0;
+}
#endif /* CONFIG_CFS_BANDWIDTH */
#endif /* CONFIG_FAIR_GROUP_SCHED */
@@ -11215,6 +11236,10 @@ static struct cftype cpu_legacy_files[] = {
.name = "stat",
.seq_show = cpu_cfs_stat_show,
},
+ {
+ .name = "stat.local",
+ .seq_show = cpu_cfs_local_stat_show,
+ },
#endif
#ifdef CONFIG_RT_GROUP_SCHED
{
@@ -11271,6 +11296,24 @@ static int cpu_extra_stat_show(struct seq_file *sf,
return 0;
}
+static int cpu_local_stat_show(struct seq_file *sf,
+ struct cgroup_subsys_state *css)
+{
+#ifdef CONFIG_CFS_BANDWIDTH
+ {
+ struct task_group *tg = css_tg(css);
+ u64 throttled_self_usec;
+
+ throttled_self_usec = throttled_time_self(tg);
+ do_div(throttled_self_usec, NSEC_PER_USEC);
+
+ seq_printf(sf, "throttled_usec %llu\n",
+ throttled_self_usec);
+ }
+#endif
+ return 0;
+}
+
#ifdef CONFIG_FAIR_GROUP_SCHED
static u64 cpu_weight_read_u64(struct cgroup_subsys_state *css,
struct cftype *cft)
@@ -11449,6 +11492,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
.css_released = cpu_cgroup_css_released,
.css_free = cpu_cgroup_css_free,
.css_extra_stat_show = cpu_extra_stat_show,
+ .css_local_stat_show = cpu_local_stat_show,
#ifdef CONFIG_RT_GROUP_SCHED
.can_attach = cpu_cgroup_can_attach,
#endif
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 51ccae7..159b202 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4878,8 +4878,12 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
list_add_leaf_cfs_rq(cfs_rq);
} else {
#ifdef CONFIG_CFS_BANDWIDTH
+ struct rq *rq = rq_of(cfs_rq);
+
if (cfs_rq_throttled(cfs_rq) && !cfs_rq->throttled_clock)
- cfs_rq->throttled_clock = rq_clock(rq_of(cfs_rq));
+ cfs_rq->throttled_clock = rq_clock(rq);
+ if (!cfs_rq->throttled_clock_self)
+ cfs_rq->throttled_clock_self = rq_clock(rq);
#endif
}
}
@@ -5384,6 +5388,17 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
/* Add cfs_rq with load or one or more already running entities to the list */
if (!cfs_rq_is_decayed(cfs_rq))
list_add_leaf_cfs_rq(cfs_rq);
+
+ if (cfs_rq->throttled_clock_self) {
+ u64 delta = rq_clock(rq) - cfs_rq->throttled_clock_self;
+
+ cfs_rq->throttled_clock_self = 0;
+
+ if (SCHED_WARN_ON((s64)delta < 0))
+ delta = 0;
+
+ cfs_rq->throttled_clock_self_time += delta;
+ }
}
return 0;
@@ -5398,6 +5413,10 @@ static int tg_throttle_down(struct task_group *tg, void *data)
if (!cfs_rq->throttle_count) {
cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
list_del_leaf_cfs_rq(cfs_rq);
+
+ SCHED_WARN_ON(cfs_rq->throttled_clock_self);
+ if (cfs_rq->nr_running)
+ cfs_rq->throttled_clock_self = rq_clock(rq);
}
cfs_rq->throttle_count++;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e93e006..1dcea9b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -636,6 +636,8 @@ struct cfs_rq {
u64 throttled_clock;
u64 throttled_clock_pelt;
u64 throttled_clock_pelt_time;
+ u64 throttled_clock_self;
+ u64 throttled_clock_self_time;
int throttled;
int throttle_count;
struct list_head throttled_list;