Re: [PATCH v2 2/2] sched/deadline: Correctly account for allocated bandwidth during hotplug

[Phil Auld <pauld@redhat.com>]

On Thu, Nov 14, 2024 at 02:28:10PM +0000 Juri Lelli wrote:
> For hotplug operations, DEADLINE needs to check that there is still enough
> bandwidth left after removing the CPU that is going offline. We however
> fail to do so currently.
> 
> Restore the correct behavior by restructuring dl_bw_manage() a bit, so
> that overflow conditions (not enough bandwidth left) are properly
> checked. Also account for dl_server bandwidth, i.e. discount such
> bandwidth in the calculation since NORMAL tasks will be anyway moved
> away from the CPU as a result of the hotplug operation.
> 
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
>

Nice, thanks!

Reviewed-by: Phil Auld <pauld@redhat.com>


> ---
> v1->v2: special case when total_bw = 0 (discounting dl_servers)
> ---
>  kernel/sched/core.c     |  2 +-
>  kernel/sched/deadline.c | 48 +++++++++++++++++++++++++++++++++--------
>  kernel/sched/sched.h    |  2 +-
>  3 files changed, 41 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 43e453ab7e20..d1049e784510 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8057,7 +8057,7 @@ static void cpuset_cpu_active(void)
>  static int cpuset_cpu_inactive(unsigned int cpu)
>  {
>  	if (!cpuhp_tasks_frozen) {
> -		int ret = dl_bw_check_overflow(cpu);
> +		int ret = dl_bw_deactivate(cpu);
>  
>  		if (ret)
>  			return ret;
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index a9cdbf058871..267ea8bacaf6 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3470,29 +3470,31 @@ int dl_cpuset_cpumask_can_shrink(const struct cpumask *cur,
>  }
>  
>  enum dl_bw_request {
> -	dl_bw_req_check_overflow = 0,
> +	dl_bw_req_deactivate = 0,
>  	dl_bw_req_alloc,
>  	dl_bw_req_free
>  };
>  
>  static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
>  {
> -	unsigned long flags;
> +	unsigned long flags, cap;
>  	struct dl_bw *dl_b;
>  	bool overflow = 0;
> +	u64 fair_server_bw = 0;
>  
>  	rcu_read_lock_sched();
>  	dl_b = dl_bw_of(cpu);
>  	raw_spin_lock_irqsave(&dl_b->lock, flags);
>  
> -	if (req == dl_bw_req_free) {
> +	cap = dl_bw_capacity(cpu);
> +	switch (req) {
> +	case dl_bw_req_free:
>  		__dl_sub(dl_b, dl_bw, dl_bw_cpus(cpu));
> -	} else {
> -		unsigned long cap = dl_bw_capacity(cpu);
> -
> +		break;
> +	case dl_bw_req_alloc:
>  		overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
>  
> -		if (req == dl_bw_req_alloc && !overflow) {
> +		if (!overflow) {
>  			/*
>  			 * We reserve space in the destination
>  			 * root_domain, as we can't fail after this point.
> @@ -3501,6 +3503,34 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
>  			 */
>  			__dl_add(dl_b, dl_bw, dl_bw_cpus(cpu));
>  		}
> +		break;
> +	case dl_bw_req_deactivate:
> +		/*
> +		 * cpu is going offline and NORMAL tasks will be moved away
> +		 * from it. We can thus discount dl_server bandwidth
> +		 * contribution as it won't need to be servicing tasks after
> +		 * the cpu is off.
> +		 */
> +		if (cpu_rq(cpu)->fair_server.dl_server)
> +			fair_server_bw = cpu_rq(cpu)->fair_server.dl_bw;
> +
> +		/*
> +		 * Not much to check if no DEADLINE bandwidth is present.
> +		 * dl_servers we can discount, as tasks will be moved out the
> +		 * offlined CPUs anyway.
> +		 */
> +		if (dl_b->total_bw - fair_server_bw > 0) {
> +			/*
> +			 * Leaving at least one CPU for DEADLINE tasks seems a
> +			 * wise thing to do.
> +			 */
> +			if (dl_bw_cpus(cpu))
> +				overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0);
> +			else
> +				overflow = 1;
> +		}
> +
> +		break;
>  	}
>  
>  	raw_spin_unlock_irqrestore(&dl_b->lock, flags);
> @@ -3509,9 +3539,9 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
>  	return overflow ? -EBUSY : 0;
>  }
>  
> -int dl_bw_check_overflow(int cpu)
> +int dl_bw_deactivate(int cpu)
>  {
> -	return dl_bw_manage(dl_bw_req_check_overflow, cpu, 0);
> +	return dl_bw_manage(dl_bw_req_deactivate, cpu, 0);
>  }
>  
>  int dl_bw_alloc(int cpu, u64 dl_bw)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b1c3588a8f00..1fee840f1bab 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -362,7 +362,7 @@ extern void __getparam_dl(struct task_struct *p, struct sched_attr *attr);
>  extern bool __checkparam_dl(const struct sched_attr *attr);
>  extern bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr);
>  extern int  dl_cpuset_cpumask_can_shrink(const struct cpumask *cur, const struct cpumask *trial);
> -extern int  dl_bw_check_overflow(int cpu);
> +extern int  dl_bw_deactivate(int cpu);
>  extern s64 dl_scaled_delta_exec(struct rq *rq, struct sched_dl_entity *dl_se, s64 delta_exec);
>  /*
>   * SCHED_DEADLINE supports servers (nested scheduling) with the following
> -- 
> 2.47.0
> 

--