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SubjectRe: [PATCH v3] sched/deadline: Fix sched_dl_global_validate()
On Sat, Sep 19, 2020 at 09:42:49AM +0800, Peng Liu wrote:
> When user changes 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.

Is there an actual problem there? Spell it out.

> 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://lkml.org/lkml/2010/2/28/119

Don't use lkml.org links, use lkml.kernel.org/r/$MsgID instead.

> [!CONFIG_SMP build error]
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Peng Liu <iwtbavbm@gmail.com>

Quite frankly this patch is horrible #ifdef soup.

Can't you make something like the below work?

---
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3862a28cd05d..3f309e0f69f5 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(i)->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
@@ -2513,31 +2529,35 @@ const struct sched_class dl_sched_class

int sched_dl_global_validate(void)
{
+ static u64 generation = 0;
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;
+ struct dl_bw *dl_b;
+ u64 gen = ++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)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 28709f6b0975..7f0947db6e2c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -801,6 +801,8 @@ 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.
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