Hi, On 26/09/20 00:20, Peng Liu wrote: > 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 <pet...@infradead.org> > Signed-off-by: Peng Liu <iwtba...@gmail.com> > --- > > 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!
So, this patch changes 2 things: it actually fixes the problem and it optimizes scanning of root domains. Even though the changes are limited, I'd be more comfortable if we split them in two (fix + optimization). Would you be up for doing it? > */ > 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; > + I think this deserves a comment explaining what it is and how it's used. Also, do we really need an u64? Maybe an smaller type would be OK as well and could fit in the 4 bytes hole that seems available after dlo_count, I'm thinking. Thanks, Juri