sched_ext currently suffers starvation due to RT. The same workload when converted to EXT can get zero runtime if RT is 100% running, causing EXT processes to stall. Fix it by adding a DL server for EXT.
A kselftest is also provided later to verify:
# ./runner -t rt_stall ===== START ===== TEST: rt_stall DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks OUTPUT: # Runtime of EXT task (PID 23338) is 0.250000 seconds # Runtime of RT task (PID 23339) is 4.750000 seconds # EXT task got 5.00% of total runtime ok 1 PASS: EXT task got more than 4.00% of runtime ===== END =====
v3: - clarify that fair is not the only dl_server (Juri Lelli) - remove explicit stop to reduce timer reprogramming overhead (Juri Lelli) - do not restart pick_task() when it's invoked by the dl_server (Tejun Heo) - depend on CONFIG_SCHED_CLASS_EXT (Andrea Righi) v2: - drop ->balance() now that pick_task() has an rf argument (Andrea Righi)
Cc: Luigi De Matteis ldematteis123@gmail.com Co-developed-by: Joel Fernandes joelagnelf@nvidia.com Signed-off-by: Joel Fernandes joelagnelf@nvidia.com Signed-off-by: Andrea Righi arighi@nvidia.com --- kernel/sched/core.c | 3 +++ kernel/sched/ext.c | 45 ++++++++++++++++++++++++++++++++++++++++++++ kernel/sched/sched.h | 2 ++ 3 files changed, 50 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 096e8d03d85e7..31a9c9381c63f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8679,6 +8679,9 @@ void __init sched_init(void) hrtick_rq_init(rq); atomic_set(&rq->nr_iowait, 0); fair_server_init(rq); +#ifdef CONFIG_SCHED_CLASS_EXT + ext_server_init(rq); +#endif
#ifdef CONFIG_SCHED_CORE rq->core = rq; diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index d1ef5bda95aec..2a25749c54ba1 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -902,6 +902,9 @@ static void update_curr_scx(struct rq *rq) if (!curr->scx.slice) touch_core_sched(rq, curr); } + + if (dl_server_active(&rq->ext_server)) + dl_server_update(&rq->ext_server, delta_exec); }
static bool scx_dsq_priq_less(struct rb_node *node_a, @@ -1409,6 +1412,15 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags if (enq_flags & SCX_ENQ_WAKEUP) touch_core_sched(rq, p);
+ if (rq->scx.nr_running == 1) { + /* Account for idle runtime */ + if (!rq->nr_running) + dl_server_update_idle_time(rq, rq->curr, &rq->ext_server); + + /* Start dl_server if this is the first task being enqueued */ + dl_server_start(&rq->ext_server); + } + do_enqueue_task(rq, p, enq_flags, sticky_cpu); out: rq->scx.flags &= ~SCX_RQ_IN_WAKEUP; @@ -2444,6 +2456,30 @@ static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf) return do_pick_task_scx(rq, rf, false); }
+/* + * Select the next task to run from the ext scheduling class. + * + * Use do_pick_task_scx() directly with @force_scx enabled, since the + * dl_server must always select a sched_ext task. + */ +static struct task_struct * +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf) +{ + return do_pick_task_scx(dl_se->rq, rf, true); +} + +/* + * Initialize the ext server deadline entity. + */ +void ext_server_init(struct rq *rq) +{ + struct sched_dl_entity *dl_se = &rq->ext_server; + + init_dl_entity(dl_se); + + dl_server_init(dl_se, rq, ext_server_pick_task); +} + #ifdef CONFIG_SCHED_CORE /** * scx_prio_less - Task ordering for core-sched @@ -3023,6 +3059,15 @@ static void switching_to_scx(struct rq *rq, struct task_struct *p) static void switched_from_scx(struct rq *rq, struct task_struct *p) { scx_disable_task(p); + + /* + * After class switch, if the DL server is still active, restart it so + * that DL timers will be queued, in case SCX switched to higher class. + */ + if (dl_server_active(&rq->ext_server)) { + dl_server_stop(&rq->ext_server); + dl_server_start(&rq->ext_server); + } }
static void wakeup_preempt_scx(struct rq *rq, struct task_struct *p,int wake_flags) {} diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index eaae470841dea..002e5c1808014 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -415,6 +415,7 @@ extern void dl_server_update_idle_time(struct rq *rq, struct task_struct *p, struct sched_dl_entity *rq_dl_server); extern void fair_server_init(struct rq *rq); +extern void ext_server_init(struct rq *rq); extern void __dl_server_attach_root(struct sched_dl_entity *dl_se, struct rq *rq); extern int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 period, bool init); @@ -1154,6 +1155,7 @@ struct rq { struct dl_rq dl; #ifdef CONFIG_SCHED_CLASS_EXT struct scx_rq scx; + struct sched_dl_entity ext_server; #endif
struct sched_dl_entity fair_server;