+// SPDX-License-Identifier: GPL-2.0
/*
* Real-Time Scheduling Class (mapped to the SCHED_FIFO and SCHED_RR
* policies)
raw_spin_unlock(&rt_b->rt_runtime_lock);
}
-#if defined(CONFIG_SMP) && defined(HAVE_RT_PUSH_IPI)
-static void push_irq_work_func(struct irq_work *work);
-#endif
-
void init_rt_rq(struct rt_rq *rt_rq)
{
struct rt_prio_array *array;
rt_rq->rt_nr_migratory = 0;
rt_rq->overloaded = 0;
plist_head_init(&rt_rq->pushable_tasks);
-
-#ifdef HAVE_RT_PUSH_IPI
- rt_rq->push_flags = 0;
- rt_rq->push_cpu = nr_cpu_ids;
- raw_spin_lock_init(&rt_rq->push_lock);
- init_irq_work(&rt_rq->push_work, push_irq_work_func);
-#endif
#endif /* CONFIG_SMP */
/* We start is dequeued state, because no RT tasks are queued */
rt_rq->rt_queued = 0;
}
#ifdef HAVE_RT_PUSH_IPI
+
/*
- * The search for the next cpu always starts at rq->cpu and ends
- * when we reach rq->cpu again. It will never return rq->cpu.
- * This returns the next cpu to check, or nr_cpu_ids if the loop
- * is complete.
+ * When a high priority task schedules out from a CPU and a lower priority
+ * task is scheduled in, a check is made to see if there's any RT tasks
+ * on other CPUs that are waiting to run because a higher priority RT task
+ * is currently running on its CPU. In this case, the CPU with multiple RT
+ * tasks queued on it (overloaded) needs to be notified that a CPU has opened
+ * up that may be able to run one of its non-running queued RT tasks.
+ *
+ * All CPUs with overloaded RT tasks need to be notified as there is currently
+ * no way to know which of these CPUs have the highest priority task waiting
+ * to run. Instead of trying to take a spinlock on each of these CPUs,
+ * which has shown to cause large latency when done on machines with many
+ * CPUs, sending an IPI to the CPUs to have them push off the overloaded
+ * RT tasks waiting to run.
+ *
+ * Just sending an IPI to each of the CPUs is also an issue, as on large
+ * count CPU machines, this can cause an IPI storm on a CPU, especially
+ * if its the only CPU with multiple RT tasks queued, and a large number
+ * of CPUs scheduling a lower priority task at the same time.
+ *
+ * Each root domain has its own irq work function that can iterate over
+ * all CPUs with RT overloaded tasks. Since all CPUs with overloaded RT
+ * tassk must be checked if there's one or many CPUs that are lowering
+ * their priority, there's a single irq work iterator that will try to
+ * push off RT tasks that are waiting to run.
+ *
+ * When a CPU schedules a lower priority task, it will kick off the
+ * irq work iterator that will jump to each CPU with overloaded RT tasks.
+ * As it only takes the first CPU that schedules a lower priority task
+ * to start the process, the rto_start variable is incremented and if
+ * the atomic result is one, then that CPU will try to take the rto_lock.
+ * This prevents high contention on the lock as the process handles all
+ * CPUs scheduling lower priority tasks.
+ *
+ * All CPUs that are scheduling a lower priority task will increment the
+ * rt_loop_next variable. This will make sure that the irq work iterator
+ * checks all RT overloaded CPUs whenever a CPU schedules a new lower
+ * priority task, even if the iterator is in the middle of a scan. Incrementing
+ * the rt_loop_next will cause the iterator to perform another scan.
*
- * rq->rt.push_cpu holds the last cpu returned by this function,
- * or if this is the first instance, it must hold rq->cpu.
*/
static int rto_next_cpu(struct rq *rq)
{
- int prev_cpu = rq->rt.push_cpu;
+ struct root_domain *rd = rq->rd;
+ int next;
int cpu;
- cpu = cpumask_next(prev_cpu, rq->rd->rto_mask);
-
/*
- * If the previous cpu is less than the rq's CPU, then it already
- * passed the end of the mask, and has started from the beginning.
- * We end if the next CPU is greater or equal to rq's CPU.
+ * When starting the IPI RT pushing, the rto_cpu is set to -1,
+ * rt_next_cpu() will simply return the first CPU found in
+ * the rto_mask.
+ *
+ * If rto_next_cpu() is called with rto_cpu is a valid cpu, it
+ * will return the next CPU found in the rto_mask.
+ *
+ * If there are no more CPUs left in the rto_mask, then a check is made
+ * against rto_loop and rto_loop_next. rto_loop is only updated with
+ * the rto_lock held, but any CPU may increment the rto_loop_next
+ * without any locking.
*/
- if (prev_cpu < rq->cpu) {
- if (cpu >= rq->cpu)
- return nr_cpu_ids;
+ for (;;) {
- } else if (cpu >= nr_cpu_ids) {
- /*
- * We passed the end of the mask, start at the beginning.
- * If the result is greater or equal to the rq's CPU, then
- * the loop is finished.
- */
- cpu = cpumask_first(rq->rd->rto_mask);
- if (cpu >= rq->cpu)
- return nr_cpu_ids;
- }
- rq->rt.push_cpu = cpu;
+ /* When rto_cpu is -1 this acts like cpumask_first() */
+ cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
- /* Return cpu to let the caller know if the loop is finished or not */
- return cpu;
-}
+ rd->rto_cpu = cpu;
-static int find_next_push_cpu(struct rq *rq)
-{
- struct rq *next_rq;
- int cpu;
+ if (cpu < nr_cpu_ids)
+ return cpu;
- while (1) {
- cpu = rto_next_cpu(rq);
- if (cpu >= nr_cpu_ids)
- break;
- next_rq = cpu_rq(cpu);
+ rd->rto_cpu = -1;
+
+ /*
+ * ACQUIRE ensures we see the @rto_mask changes
+ * made prior to the @next value observed.
+ *
+ * Matches WMB in rt_set_overload().
+ */
+ next = atomic_read_acquire(&rd->rto_loop_next);
- /* Make sure the next rq can push to this rq */
- if (next_rq->rt.highest_prio.next < rq->rt.highest_prio.curr)
+ if (rd->rto_loop == next)
break;
+
+ rd->rto_loop = next;
}
- return cpu;
+ return -1;
}
-#define RT_PUSH_IPI_EXECUTING 1
-#define RT_PUSH_IPI_RESTART 2
+static inline bool rto_start_trylock(atomic_t *v)
+{
+ return !atomic_cmpxchg_acquire(v, 0, 1);
+}
-/*
- * When a high priority task schedules out from a CPU and a lower priority
- * task is scheduled in, a check is made to see if there's any RT tasks
- * on other CPUs that are waiting to run because a higher priority RT task
- * is currently running on its CPU. In this case, the CPU with multiple RT
- * tasks queued on it (overloaded) needs to be notified that a CPU has opened
- * up that may be able to run one of its non-running queued RT tasks.
- *
- * On large CPU boxes, there's the case that several CPUs could schedule
- * a lower priority task at the same time, in which case it will look for
- * any overloaded CPUs that it could pull a task from. To do this, the runqueue
- * lock must be taken from that overloaded CPU. Having 10s of CPUs all fighting
- * for a single overloaded CPU's runqueue lock can produce a large latency.
- * (This has actually been observed on large boxes running cyclictest).
- * Instead of taking the runqueue lock of the overloaded CPU, each of the
- * CPUs that scheduled a lower priority task simply sends an IPI to the
- * overloaded CPU. An IPI is much cheaper than taking an runqueue lock with
- * lots of contention. The overloaded CPU will look to push its non-running
- * RT task off, and if it does, it can then ignore the other IPIs coming
- * in, and just pass those IPIs off to any other overloaded CPU.
- *
- * When a CPU schedules a lower priority task, it only sends an IPI to
- * the "next" CPU that has overloaded RT tasks. This prevents IPI storms,
- * as having 10 CPUs scheduling lower priority tasks and 10 CPUs with
- * RT overloaded tasks, would cause 100 IPIs to go out at once.
- *
- * The overloaded RT CPU, when receiving an IPI, will try to push off its
- * overloaded RT tasks and then send an IPI to the next CPU that has
- * overloaded RT tasks. This stops when all CPUs with overloaded RT tasks
- * have completed. Just because a CPU may have pushed off its own overloaded
- * RT task does not mean it should stop sending the IPI around to other
- * overloaded CPUs. There may be another RT task waiting to run on one of
- * those CPUs that are of higher priority than the one that was just
- * pushed.
- *
- * An optimization that could possibly be made is to make a CPU array similar
- * to the cpupri array mask of all running RT tasks, but for the overloaded
- * case, then the IPI could be sent to only the CPU with the highest priority
- * RT task waiting, and that CPU could send off further IPIs to the CPU with
- * the next highest waiting task. Since the overloaded case is much less likely
- * to happen, the complexity of this implementation may not be worth it.
- * Instead, just send an IPI around to all overloaded CPUs.
- *
- * The rq->rt.push_flags holds the status of the IPI that is going around.
- * A run queue can only send out a single IPI at a time. The possible flags
- * for rq->rt.push_flags are:
- *
- * (None or zero): No IPI is going around for the current rq
- * RT_PUSH_IPI_EXECUTING: An IPI for the rq is being passed around
- * RT_PUSH_IPI_RESTART: The priority of the running task for the rq
- * has changed, and the IPI should restart
- * circulating the overloaded CPUs again.
- *
- * rq->rt.push_cpu contains the CPU that is being sent the IPI. It is updated
- * before sending to the next CPU.
- *
- * Instead of having all CPUs that schedule a lower priority task send
- * an IPI to the same "first" CPU in the RT overload mask, they send it
- * to the next overloaded CPU after their own CPU. This helps distribute
- * the work when there's more than one overloaded CPU and multiple CPUs
- * scheduling in lower priority tasks.
- *
- * When a rq schedules a lower priority task than what was currently
- * running, the next CPU with overloaded RT tasks is examined first.
- * That is, if CPU 1 and 5 are overloaded, and CPU 3 schedules a lower
- * priority task, it will send an IPI first to CPU 5, then CPU 5 will
- * send to CPU 1 if it is still overloaded. CPU 1 will clear the
- * rq->rt.push_flags if RT_PUSH_IPI_RESTART is not set.
- *
- * The first CPU to notice IPI_RESTART is set, will clear that flag and then
- * send an IPI to the next overloaded CPU after the rq->cpu and not the next
- * CPU after push_cpu. That is, if CPU 1, 4 and 5 are overloaded when CPU 3
- * schedules a lower priority task, and the IPI_RESTART gets set while the
- * handling is being done on CPU 5, it will clear the flag and send it back to
- * CPU 4 instead of CPU 1.
- *
- * Note, the above logic can be disabled by turning off the sched_feature
- * RT_PUSH_IPI. Then the rq lock of the overloaded CPU will simply be
- * taken by the CPU requesting a pull and the waiting RT task will be pulled
- * by that CPU. This may be fine for machines with few CPUs.
- */
-static void tell_cpu_to_push(struct rq *rq)
+static inline void rto_start_unlock(atomic_t *v)
{
- int cpu;
+ atomic_set_release(v, 0);
+}
- if (rq->rt.push_flags & RT_PUSH_IPI_EXECUTING) {
- raw_spin_lock(&rq->rt.push_lock);
- /* Make sure it's still executing */
- if (rq->rt.push_flags & RT_PUSH_IPI_EXECUTING) {
- /*
- * Tell the IPI to restart the loop as things have
- * changed since it started.
- */
- rq->rt.push_flags |= RT_PUSH_IPI_RESTART;
- raw_spin_unlock(&rq->rt.push_lock);
- return;
- }
- raw_spin_unlock(&rq->rt.push_lock);
- }
+static void tell_cpu_to_push(struct rq *rq)
+{
+ int cpu = -1;
- /* When here, there's no IPI going around */
+ /* Keep the loop going if the IPI is currently active */
+ atomic_inc(&rq->rd->rto_loop_next);
- rq->rt.push_cpu = rq->cpu;
- cpu = find_next_push_cpu(rq);
- if (cpu >= nr_cpu_ids)
+ /* Only one CPU can initiate a loop at a time */
+ if (!rto_start_trylock(&rq->rd->rto_loop_start))
return;
- rq->rt.push_flags = RT_PUSH_IPI_EXECUTING;
+ raw_spin_lock(&rq->rd->rto_lock);
+
+ /*
+ * The rto_cpu is updated under the lock, if it has a valid cpu
+ * then the IPI is still running and will continue due to the
+ * update to loop_next, and nothing needs to be done here.
+ * Otherwise it is finishing up and an ipi needs to be sent.
+ */
+ if (rq->rd->rto_cpu < 0)
+ cpu = rto_next_cpu(rq);
- irq_work_queue_on(&rq->rt.push_work, cpu);
+ raw_spin_unlock(&rq->rd->rto_lock);
+
+ rto_start_unlock(&rq->rd->rto_loop_start);
+
+ if (cpu >= 0)
+ irq_work_queue_on(&rq->rd->rto_push_work, cpu);
}
/* Called from hardirq context */
-static void try_to_push_tasks(void *arg)
+void rto_push_irq_work_func(struct irq_work *work)
{
- struct rt_rq *rt_rq = arg;
- struct rq *rq, *src_rq;
- int this_cpu;
+ struct rq *rq;
int cpu;
- this_cpu = rt_rq->push_cpu;
+ rq = this_rq();
- /* Paranoid check */
- BUG_ON(this_cpu != smp_processor_id());
-
- rq = cpu_rq(this_cpu);
- src_rq = rq_of_rt_rq(rt_rq);
-
-again:
+ /*
+ * We do not need to grab the lock to check for has_pushable_tasks.
+ * When it gets updated, a check is made if a push is possible.
+ */
if (has_pushable_tasks(rq)) {
raw_spin_lock(&rq->lock);
- push_rt_task(rq);
+ push_rt_tasks(rq);
raw_spin_unlock(&rq->lock);
}
- /* Pass the IPI to the next rt overloaded queue */
- raw_spin_lock(&rt_rq->push_lock);
- /*
- * If the source queue changed since the IPI went out,
- * we need to restart the search from that CPU again.
- */
- if (rt_rq->push_flags & RT_PUSH_IPI_RESTART) {
- rt_rq->push_flags &= ~RT_PUSH_IPI_RESTART;
- rt_rq->push_cpu = src_rq->cpu;
- }
+ raw_spin_lock(&rq->rd->rto_lock);
- cpu = find_next_push_cpu(src_rq);
+ /* Pass the IPI to the next rt overloaded queue */
+ cpu = rto_next_cpu(rq);
- if (cpu >= nr_cpu_ids)
- rt_rq->push_flags &= ~RT_PUSH_IPI_EXECUTING;
- raw_spin_unlock(&rt_rq->push_lock);
+ raw_spin_unlock(&rq->rd->rto_lock);
- if (cpu >= nr_cpu_ids)
+ if (cpu < 0)
return;
- /*
- * It is possible that a restart caused this CPU to be
- * chosen again. Don't bother with an IPI, just see if we
- * have more to push.
- */
- if (unlikely(cpu == rq->cpu))
- goto again;
-
/* Try the next RT overloaded CPU */
- irq_work_queue_on(&rt_rq->push_work, cpu);
-}
-
-static void push_irq_work_func(struct irq_work *work)
-{
- struct rt_rq *rt_rq = container_of(work, struct rt_rq, push_work);
-
- try_to_push_tasks(rt_rq);
+ irq_work_queue_on(&rq->rd->rto_push_work, cpu);
}
#endif /* HAVE_RT_PUSH_IPI */
projects / linux-2.6-microblaze.git / blobdiff