[PATCH v4] sched/deadline: Fix sched_dl_global_validate()

From: Peng Liu
Date: Fri Sep 25 2020 - 12:20:39 EST

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I created another root domain(contains 2 CPUs) besides the default
one, and the global default rt bandwidth is 95%. Then launched a
DL process which need 25% bandwidth and moved it to the new root
domain, so far so good.

Then I tried to change global rt bandwidth to 20% with cmd:
echo 200000 > /proc/sys/kernel/sched_rt_runtime_us
but ending with the below error:
bash: echo: write error: Device or resource busy
Only values greater than 250000 could work.

The new root domain contains two CPUs, thus should could provide
totally 2*20%(>25%) bandwidth. So the error is strange.
Finally I found it's the sched_dl_global_validate() mistakenly
do the validation.

When change sched_rt_{runtime, period}_us, then

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;
}
}

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.
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, correct 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")
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Peng Liu <iwtbavbm@xxxxxxxxx>
---

v4 <-- v3:
- refine changelog;
- eliminate the ugly #ifdef guys with Peter's method;

v3 <-- v2:
- fix build error for !CONFIG_SMP, reported by kernel test robot;

v2 <-- v1:
- replace cpumask_weight(cpu_rq(cpu)->rd->span) with dl_bw_cpus(cpu),
suggested by Juri;

kernel/sched/deadline.c | 43 ++++++++++++++++++++++++++++++++---------
kernel/sched/sched.h | 22 +++++++++------------
kernel/sched/topology.c | 1 +
3 files changed, 44 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3862a28cd05d..c95af33b7274 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -97,6 +97,17 @@ static inline unsigned long dl_bw_capacity(int i)
return __dl_bw_capacity(i);
}
}
+
+static inline bool dl_bw_visited(int cpu, u64 gen)
+{
+ struct root_domain *rd = cpu_rq(cpu)->rd;
+
+ if (rd->visit_gen == gen)
+ return true;
+
+ rd->visit_gen = gen;
+ return false;
+}
#else
static inline struct dl_bw *dl_bw_of(int i)
{
@@ -112,6 +123,11 @@ static inline unsigned long dl_bw_capacity(int i)
{
return SCHED_CAPACITY_SCALE;
}
+
+static inline bool dl_bw_visited(int cpu, u64 gen)
+{
+ return false;
+}
#endif

static inline
@@ -2511,33 +2527,38 @@ const struct sched_class dl_sched_class
.update_curr = update_curr_dl,
};

+static u64 dl_generation;
+
int sched_dl_global_validate(void)
{
u64 runtime = global_rt_runtime();
u64 period = global_rt_period();
u64 new_bw = to_ratio(period, runtime);
struct dl_bw *dl_b;
- int cpu, ret = 0;
+ int cpu, cpus, ret = 0;
unsigned long flags;
+ u64 gen = ++dl_generation;

/*
* Here we want to check the bandwidth not being set to some
* value smaller than the currently allocated bandwidth in
* any of the root_domains.
- *
- * FIXME: Cycling on all the CPUs is overdoing, but simpler than
- * cycling on root_domains... Discussion on different/better
- * solutions is welcome!
*/
for_each_possible_cpu(cpu) {
rcu_read_lock_sched();
+
+ if (dl_bw_visited(cpu, gen))
+ 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);

+next:
rcu_read_unlock_sched();

if (ret)
@@ -2563,6 +2584,7 @@ static void init_dl_rq_bw_ratio(struct dl_rq *dl_rq)
void sched_dl_do_global(void)
{
u64 new_bw = -1;
+ u64 gen = ++dl_generation;
struct dl_bw *dl_b;
int cpu;
unsigned long flags;
@@ -2573,11 +2595,14 @@ void sched_dl_do_global(void)
if (global_rt_runtime() != RUNTIME_INF)
new_bw = to_ratio(global_rt_period(), global_rt_runtime());

- /*
- * FIXME: As above...
- */
for_each_possible_cpu(cpu) {
rcu_read_lock_sched();
+
+ if (dl_bw_visited(cpu, gen)) {
+ rcu_read_unlock_sched();
+ continue;
+ }
+
dl_b = dl_bw_of(cpu);

raw_spin_lock_irqsave(&dl_b->lock, flags);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 28709f6b0975..c23976bc38e9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -258,10 +258,11 @@ struct rt_bandwidth {
void __dl_clear_params(struct task_struct *p);

/*
- * To keep the bandwidth of -deadline tasks and groups under control
+ * To keep the bandwidth of -deadline tasks 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.
+ * - store the maximum -deadline bandwidth of each CPU;
+ * - cache the fraction of that 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
@@ -269,17 +270,10 @@ void __dl_clear_params(struct task_struct *p);
* 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,
+ * With respect to SMP, the bandwidth is given on per root domain 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.
+ * - bw (< 100%) is the deadline bandwidth of each CPU;
+ * - total_bw is the currently allocated bandwidth on each root domain.
*/
struct dl_bandwidth {
raw_spinlock_t dl_runtime_lock;
@@ -801,6 +795,8 @@ struct root_domain {
struct dl_bw dl_bw;
struct cpudl cpudl;

+ u64 visit_gen;
+
#ifdef HAVE_RT_PUSH_IPI
/*
* For IPI pull requests, loop across the rto_mask.
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 55c453d140e9..f431daefed68 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -516,6 +516,7 @@ static int init_rootdomain(struct root_domain *rd)
init_irq_work(&rd->rto_push_work, rto_push_irq_work_func);
#endif

+ rd->visit_gen = 0;
init_dl_bw(&rd->dl_bw);
if (cpudl_init(&rd->cpudl) != 0)
goto free_rto_mask;
--
2.20.1

Next message: Hui Su: "[PATCH] mm: fix some comments in page_alloc.c and mempolicy.c"
Previous message: Christoph Hellwig: "Re: [PATCH 1/8] dma-mapping: add DMA_ATTR_LOW_ADDRESS attribute"
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