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authorPhil Auld <pauld@redhat.com>2023-07-12 16:33:57 +0300
committerPeter Zijlstra <peterz@infradead.org>2023-08-02 17:19:26 +0300
commit88c56cfeaec4642aee8aac58b38d5708c6aae0d3 (patch)
treea931bedfa512ff5828f15bf56d50158b030a1b60 /kernel/sched
parentc98c18270be115678f4295b10a5af5dcc9c4efa0 (diff)
downloadlinux-88c56cfeaec4642aee8aac58b38d5708c6aae0d3.tar.xz
sched/fair: Block nohz tick_stop when cfs bandwidth in use
CFS bandwidth limits and NOHZ full don't play well together. Tasks can easily run well past their quotas before a remote tick does accounting. This leads to long, multi-period stalls before such tasks can run again. Currently, when presented with these conflicting requirements the scheduler is favoring nohz_full and letting the tick be stopped. However, nohz tick stopping is already best-effort, there are a number of conditions that can prevent it, whereas cfs runtime bandwidth is expected to be enforced. Make the scheduler favor bandwidth over stopping the tick by setting TICK_DEP_BIT_SCHED when the only running task is a cfs task with runtime limit enabled. We use cfs_b->hierarchical_quota to determine if the task requires the tick. Add check in pick_next_task_fair() as well since that is where we have a handle on the task that is actually going to be running. Add check in sched_can_stop_tick() to cover some edge cases such as nr_running going from 2->1 and the 1 remains the running task. Reviewed-By: Ben Segall <bsegall@google.com> Signed-off-by: Phil Auld <pauld@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20230712133357.381137-3-pauld@redhat.com
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c26
-rw-r--r--kernel/sched/fair.c52
-rw-r--r--kernel/sched/features.h2
-rw-r--r--kernel/sched/sched.h2
4 files changed, 81 insertions, 1 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3af25caf6343..614271a75525 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1194,6 +1194,20 @@ static void nohz_csd_func(void *info)
#endif /* CONFIG_NO_HZ_COMMON */
#ifdef CONFIG_NO_HZ_FULL
+static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
+{
+ if (rq->nr_running != 1)
+ return false;
+
+ if (p->sched_class != &fair_sched_class)
+ return false;
+
+ if (!task_on_rq_queued(p))
+ return false;
+
+ return true;
+}
+
bool sched_can_stop_tick(struct rq *rq)
{
int fifo_nr_running;
@@ -1229,6 +1243,18 @@ bool sched_can_stop_tick(struct rq *rq)
if (rq->nr_running > 1)
return false;
+ /*
+ * If there is one task and it has CFS runtime bandwidth constraints
+ * and it's on the cpu now we don't want to stop the tick.
+ * This check prevents clearing the bit if a newly enqueued task here is
+ * dequeued by migrating while the constrained task continues to run.
+ * E.g. going from 2->1 without going through pick_next_task().
+ */
+ if (sched_feat(HZ_BW) && __need_bw_check(rq, rq->curr)) {
+ if (cfs_task_bw_constrained(rq->curr))
+ return false;
+ }
+
return true;
}
#endif /* CONFIG_NO_HZ_FULL */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 26bfbb640894..c28206499a3d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6189,6 +6189,46 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
rq_clock_stop_loop_update(rq);
}
+bool cfs_task_bw_constrained(struct task_struct *p)
+{
+ struct cfs_rq *cfs_rq = task_cfs_rq(p);
+
+ if (!cfs_bandwidth_used())
+ return false;
+
+ if (cfs_rq->runtime_enabled ||
+ tg_cfs_bandwidth(cfs_rq->tg)->hierarchical_quota != RUNTIME_INF)
+ return true;
+
+ return false;
+}
+
+#ifdef CONFIG_NO_HZ_FULL
+/* called from pick_next_task_fair() */
+static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
+{
+ int cpu = cpu_of(rq);
+
+ if (!sched_feat(HZ_BW) || !cfs_bandwidth_used())
+ return;
+
+ if (!tick_nohz_full_cpu(cpu))
+ return;
+
+ if (rq->nr_running != 1)
+ return;
+
+ /*
+ * We know there is only one task runnable and we've just picked it. The
+ * normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
+ * be otherwise able to stop the tick. Just need to check if we are using
+ * bandwidth control.
+ */
+ if (cfs_task_bw_constrained(p))
+ tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
+}
+#endif
+
#else /* CONFIG_CFS_BANDWIDTH */
static inline bool cfs_bandwidth_used(void)
@@ -6231,9 +6271,18 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
static inline void update_runtime_enabled(struct rq *rq) {}
static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
-
+#ifdef CONFIG_CGROUP_SCHED
+bool cfs_task_bw_constrained(struct task_struct *p)
+{
+ return false;
+}
+#endif
#endif /* CONFIG_CFS_BANDWIDTH */
+#if !defined(CONFIG_CFS_BANDWIDTH) || !defined(CONFIG_NO_HZ_FULL)
+static inline void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) {}
+#endif
+
/**************************************************
* CFS operations on tasks:
*/
@@ -8201,6 +8250,7 @@ done: __maybe_unused;
hrtick_start_fair(rq, p);
update_misfit_status(p, rq);
+ sched_fair_update_stop_tick(rq, p);
return p;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..e10074cb4be4 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false)
SCHED_FEAT(ALT_PERIOD, true)
SCHED_FEAT(BASE_SLICE, true)
+
+SCHED_FEAT(HZ_BW, true)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 602de71b48e1..19af1766df2d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -459,6 +459,7 @@ extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth
extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
+extern bool cfs_task_bw_constrained(struct task_struct *p);
extern void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
struct sched_rt_entity *rt_se, int cpu,
@@ -494,6 +495,7 @@ static inline void set_task_rq_fair(struct sched_entity *se,
#else /* CONFIG_CGROUP_SCHED */
struct cfs_bandwidth { };
+static inline bool cfs_task_bw_constrained(struct task_struct *p) { return false; }
#endif /* CONFIG_CGROUP_SCHED */