[PATCH v6 2/2] sched/deadline: Fix sched_dl_global_validate()

From: Peng Liu
Date: Thu Oct 08 2020 - 11:49:50 EST


When change sched_rt_{runtime, period}_us, we validate that the new
settings should at least accommodate the currently allocated -dl
bandwidth:

sched_rt_handler()
--> sched_dl_bandwidth_validate()
{
new_bw = global_rt_runtime()/global_rt_period();

for_each_possible_cpu(cpu) {
dl_b = dl_bw_of(cpu);
if (new_bw < dl_b->total_bw) <-------
ret = -EBUSY;
}
}

But under CONFIG_SMP, dl_bw is per root domain , but not per CPU,
dl_b->total_bw is the allocated bandwidth of the whole root domain.
Instead, we should compare dl_b->total_bw against "cpus*new_bw",
where 'cpus' is the number of CPUs of the root domain.

Also, below annotation(in kernel/sched/sched.h) implied implementation
only appeared in SCHED_DEADLINE v2[1], then deadline scheduler kept
evolving till got merged(v9), but the annotation remains unchanged,
meaningless and misleading, update it.

* With respect to SMP, the bandwidth is given on a per-CPU basis,
* meaning that:
* - dl_bw (< 100%) is the bandwidth of the system (group) on each CPU;
* - dl_total_bw array contains, in the i-eth element, the currently
* allocated bandwidth on the i-eth CPU.

[1]: https://lore.kernel.org/lkml/1267385230.13676.101.camel@Palantir/

Fixes: 332ac17ef5bf ("sched/deadline: Add bandwidth management for SCHED_DEADLINE tasks")
Signed-off-by: Peng Liu <iwtbavbm@xxxxxxxxx>
---
kernel/sched/deadline.c | 5 +++--
kernel/sched/sched.h | 42 ++++++++++++++++++-----------------------
2 files changed, 21 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 365e5fec8c28..ac0c53672e83 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2540,7 +2540,7 @@ int sched_dl_global_validate(void)
u64 new_bw = to_ratio(period, runtime);
u64 gen = ++dl_generation;
struct dl_bw *dl_b;
- int cpu, ret = 0;
+ int cpu, cpus, ret = 0;
unsigned long flags;

/*
@@ -2555,9 +2555,10 @@ int sched_dl_global_validate(void)
goto next;

dl_b = dl_bw_of(cpu);
+ cpus = dl_bw_cpus(cpu);

raw_spin_lock_irqsave(&dl_b->lock, flags);
- if (new_bw < dl_b->total_bw)
+ if (new_bw * cpus < dl_b->total_bw)
ret = -EBUSY;
raw_spin_unlock_irqrestore(&dl_b->lock, flags);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ce527b981e61..5076ea05b2e0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -257,30 +257,6 @@ struct rt_bandwidth {

void __dl_clear_params(struct task_struct *p);

-/*
- * To keep the bandwidth of -deadline tasks and groups under control
- * we need some place where:
- * - store the maximum -deadline bandwidth of the system (the group);
- * - cache the fraction of that bandwidth that is currently allocated.
- *
- * This is all done in the data structure below. It is similar to the
- * one used for RT-throttling (rt_bandwidth), with the main difference
- * that, since here we are only interested in admission control, we
- * do not decrease any runtime while the group "executes", neither we
- * need a timer to replenish it.
- *
- * With respect to SMP, the bandwidth is given on a per-CPU basis,
- * meaning that:
- * - dl_bw (< 100%) is the bandwidth of the system (group) on each CPU;
- * - dl_total_bw array contains, in the i-eth element, the currently
- * allocated bandwidth on the i-eth CPU.
- * Moreover, groups consume bandwidth on each CPU, while tasks only
- * consume bandwidth on the CPU they're running on.
- * Finally, dl_total_bw_cpu is used to cache the index of dl_total_bw
- * that will be shown the next time the proc or cgroup controls will
- * be red. It on its turn can be changed by writing on its own
- * control.
- */
struct dl_bandwidth {
raw_spinlock_t dl_runtime_lock;
u64 dl_runtime;
@@ -292,6 +268,24 @@ static inline int dl_bandwidth_enabled(void)
return sysctl_sched_rt_runtime >= 0;
}

+/*
+ * To keep the bandwidth of -deadline tasks under control
+ * we need some place where:
+ * - store the maximum -deadline bandwidth of each cpu;
+ * - cache the fraction of bandwidth that is currently allocated in
+ * each root domain;
+ *
+ * This is all done in the data structure below. It is similar to the
+ * one used for RT-throttling (rt_bandwidth), with the main difference
+ * that, since here we are only interested in admission control, we
+ * do not decrease any runtime while the group "executes", neither we
+ * need a timer to replenish it.
+ *
+ * With respect to SMP, bandwidth is given on a per root domain basis,
+ * meaning that:
+ * - bw (< 100%) is the deadline bandwidth of each CPU;
+ * - total_bw is the currently allocated bandwidth in each root domain;
+ */
struct dl_bw {
raw_spinlock_t lock;
u64 bw;
--
2.20.1