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 included later to confirm that both DL servers are functioning correctly:
# ./runner -t rt_stall ===== START ===== TEST: rt_stall DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks OUTPUT: TAP version 13 1..1 # Runtime of FAIR task (PID 1511) is 0.250000 seconds # Runtime of RT task (PID 1512) is 4.750000 seconds # FAIR task got 5.00% of total runtime ok 1 PASS: FAIR task got more than 4.00% of runtime TAP version 13 1..1 # Runtime of EXT task (PID 1514) is 0.250000 seconds # Runtime of RT task (PID 1515) is 4.750000 seconds # EXT task got 5.00% of total runtime ok 2 PASS: EXT task got more than 4.00% of runtime TAP version 13 1..1 # Runtime of FAIR task (PID 1517) is 0.250000 seconds # Runtime of RT task (PID 1518) is 4.750000 seconds # FAIR task got 5.00% of total runtime ok 3 PASS: FAIR task got more than 4.00% of runtime TAP version 13 1..1 # Runtime of EXT task (PID 1521) is 0.250000 seconds # Runtime of RT task (PID 1522) is 4.750000 seconds # EXT task got 5.00% of total runtime ok 4 PASS: EXT task got more than 4.00% of runtime ok 1 rt_stall # ===== END =====
v5: - do not restart the EXT server on switch_class() (Juri Lelli) v4: - initialize EXT server bandwidth reservation at init time and always keep it active (Andrea Righi) - check for rq->nr_running == 1 to determine when to account idle time (Juri Lelli) 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)
Tested-by: Christian Loehle christian.loehle@arm.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 | 6 +++ kernel/sched/deadline.c | 84 ++++++++++++++++++++++++++++++----------- kernel/sched/ext.c | 33 ++++++++++++++++ kernel/sched/idle.c | 3 ++ kernel/sched/sched.h | 2 + kernel/sched/topology.c | 5 +++ 6 files changed, 110 insertions(+), 23 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 41ba0be169117..a2400ee33a356 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8475,6 +8475,9 @@ int sched_cpu_dying(unsigned int cpu) dump_rq_tasks(rq, KERN_WARNING); } dl_server_stop(&rq->fair_server); +#ifdef CONFIG_SCHED_CLASS_EXT + dl_server_stop(&rq->ext_server); +#endif rq_unlock_irqrestore(rq, &rf);
calc_load_migrate(rq); @@ -8678,6 +8681,9 @@ 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/deadline.c b/kernel/sched/deadline.c index 2789db5217cd4..88f2b5ed5678a 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1445,8 +1445,8 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 dl_se->dl_defer_idle = 0;
/* - * The fair server can consume its runtime while throttled (not queued/ - * running as regular CFS). + * The DL server can consume its runtime while throttled (not + * queued / running as regular CFS). * * If the server consumes its entire runtime in this state. The server * is not required for the current period. Thus, reset the server by @@ -1531,10 +1531,10 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 }
/* - * The fair server (sole dl_server) does not account for real-time - * workload because it is running fair work. + * The dl_server does not account for real-time workload because it + * is running fair work. */ - if (dl_se == &rq->fair_server) + if (dl_se->dl_server) return;
#ifdef CONFIG_RT_GROUP_SCHED @@ -1569,9 +1569,9 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 * In the non-defer mode, the idle time is not accounted, as the * server provides a guarantee. * - * If the dl_server is in defer mode, the idle time is also considered - * as time available for the fair server, avoiding a penalty for the - * rt scheduler that did not consumed that time. + * If the dl_server is in defer mode, the idle time is also considered as + * time available for the dl_server, avoiding a penalty for the rt + * scheduler that did not consumed that time. */ void dl_server_update_idle(struct sched_dl_entity *dl_se, s64 delta_exec) { @@ -1810,6 +1810,7 @@ void dl_server_stop(struct sched_dl_entity *dl_se) hrtimer_try_to_cancel(&dl_se->dl_timer); dl_se->dl_defer_armed = 0; dl_se->dl_throttled = 0; + dl_se->dl_defer_running = 0; dl_se->dl_defer_idle = 0; dl_se->dl_server_active = 0; } @@ -1844,6 +1845,18 @@ void sched_init_dl_servers(void) dl_se->dl_server = 1; dl_se->dl_defer = 1; setup_new_dl_entity(dl_se); + +#ifdef CONFIG_SCHED_CLASS_EXT + dl_se = &rq->ext_server; + + WARN_ON(dl_server(dl_se)); + + dl_server_apply_params(dl_se, runtime, period, 1); + + dl_se->dl_server = 1; + dl_se->dl_defer = 1; + setup_new_dl_entity(dl_se); +#endif } }
@@ -3183,6 +3196,36 @@ void dl_add_task_root_domain(struct task_struct *p) raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags); }
+static void dl_server_add_bw(struct root_domain *rd, int cpu) +{ + struct sched_dl_entity *dl_se; + + dl_se = &cpu_rq(cpu)->fair_server; + if (dl_server(dl_se) && cpu_active(cpu)) + __dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(cpu)); + +#ifdef CONFIG_SCHED_CLASS_EXT + dl_se = &cpu_rq(cpu)->ext_server; + if (dl_server(dl_se) && cpu_active(cpu)) + __dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(cpu)); +#endif +} + +static u64 dl_server_read_bw(int cpu) +{ + u64 dl_bw = 0; + + if (cpu_rq(cpu)->fair_server.dl_server) + dl_bw += cpu_rq(cpu)->fair_server.dl_bw; + +#ifdef CONFIG_SCHED_CLASS_EXT + if (cpu_rq(cpu)->ext_server.dl_server) + dl_bw += cpu_rq(cpu)->ext_server.dl_bw; +#endif + + return dl_bw; +} + void dl_clear_root_domain(struct root_domain *rd) { int i; @@ -3201,12 +3244,8 @@ void dl_clear_root_domain(struct root_domain *rd) * dl_servers are not tasks. Since dl_add_task_root_domain ignores * them, we need to account for them here explicitly. */ - for_each_cpu(i, rd->span) { - struct sched_dl_entity *dl_se = &cpu_rq(i)->fair_server; - - if (dl_server(dl_se) && cpu_active(i)) - __dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(i)); - } + for_each_cpu(i, rd->span) + dl_server_add_bw(rd, i); }
void dl_clear_root_domain_cpu(int cpu) @@ -3702,7 +3741,7 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw) unsigned long flags, cap; struct dl_bw *dl_b; bool overflow = 0; - u64 fair_server_bw = 0; + u64 dl_server_bw = 0;
rcu_read_lock_sched(); dl_b = dl_bw_of(cpu); @@ -3735,27 +3774,26 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw) cap -= arch_scale_cpu_capacity(cpu);
/* - * cpu is going offline and NORMAL tasks will be moved away - * from it. We can thus discount dl_server bandwidth - * contribution as it won't need to be servicing tasks after - * the cpu is off. + * cpu is going offline and NORMAL and EXT tasks will be + * moved away from it. We can thus discount dl_server + * bandwidth contribution as it won't need to be servicing + * tasks after the cpu is off. */ - if (cpu_rq(cpu)->fair_server.dl_server) - fair_server_bw = cpu_rq(cpu)->fair_server.dl_bw; + dl_server_bw = dl_server_read_bw(cpu);
/* * Not much to check if no DEADLINE bandwidth is present. * dl_servers we can discount, as tasks will be moved out the * offlined CPUs anyway. */ - if (dl_b->total_bw - fair_server_bw > 0) { + if (dl_b->total_bw - dl_server_bw > 0) { /* * Leaving at least one CPU for DEADLINE tasks seems a * wise thing to do. As said above, cpu is not offline * yet, so account for that. */ if (dl_bw_cpus(cpu) - 1) - overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0); + overflow = __dl_overflow(dl_b, cap, dl_server_bw, 0); else overflow = 1; } diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 94164f2dec6dc..d8c457c853aef 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -957,6 +957,8 @@ static void update_curr_scx(struct rq *rq) if (!curr->scx.slice) touch_core_sched(rq, curr); } + + dl_server_update(&rq->ext_server, delta_exec); }
static bool scx_dsq_priq_less(struct rb_node *node_a, @@ -1500,6 +1502,10 @@ 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);
+ /* Start dl_server if this is the first task being enqueued */ + if (rq->scx.nr_running == 1) + dl_server_start(&rq->ext_server); + do_enqueue_task(rq, p, enq_flags, sticky_cpu); out: rq->scx.flags &= ~SCX_RQ_IN_WAKEUP; @@ -2511,6 +2517,33 @@ 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) +{ + if (!scx_enabled()) + return NULL; + + 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 diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index c174afe1dd177..53793b9a04185 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -530,6 +530,9 @@ static void update_curr_idle(struct rq *rq) se->exec_start = now;
dl_server_update_idle(&rq->fair_server, delta_exec); +#ifdef CONFIG_SCHED_CLASS_EXT + dl_server_update_idle(&rq->ext_server, delta_exec); +#endif }
/* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index d30cca6870f5f..28c24cda1c3ce 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -414,6 +414,7 @@ extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq, extern void sched_init_dl_servers(void);
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); @@ -1151,6 +1152,7 @@ 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; diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index cf643a5ddedd2..ac268da917781 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -508,6 +508,11 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd) if (rq->fair_server.dl_server) __dl_server_attach_root(&rq->fair_server, rq);
+#ifdef CONFIG_SCHED_CLASS_EXT + if (rq->ext_server.dl_server) + __dl_server_attach_root(&rq->ext_server, rq); +#endif + rq_unlock_irqrestore(rq, &rf);
if (old_rd)