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Date: Wed, 6 Mar 2024 16:46:38 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Shrikanth Hegde <sshegde@...ux.ibm.com>,
Peter Zijlstra <peterz@...radead.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
Linus Torvalds <torvalds@...ux-foundation.org>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH 3/9] sched/balancing: Change 'enum cpu_idle_type' to have
more natural definitions
On Mon, 4 Mar 2024 at 10:48, Ingo Molnar <mingo@...nel.org> wrote:
>
> The cpu_idle_type enum has the confusingly inverted property
> that 'not idle' is 1, and 'idle' is '0'.
>
> This resulted in a number of unnecessary complications in the code.
>
> Reverse the order, remove the CPU_NOT_IDLE type, and convert
> all code to a natural boolean form.
>
> It's much more readable:
>
> - enum cpu_idle_type idle = this_rq->idle_balance ?
> - CPU_IDLE : CPU_NOT_IDLE;
> -
> + enum cpu_idle_type idle = this_rq->idle_balance;
>
> --------------------------------
>
> - if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
> + if (!env->idle || !busiest->sum_nr_running)
>
> --------------------------------
>
> And gets rid of the double negation in these usages:
>
> - if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
> + if (env->idle && env->src_rq->nr_running <= 1)
>
> Furthermore, this makes code much more obvious where there's
> differentiation between CPU_IDLE and CPU_NEWLY_IDLE.
>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Valentin Schneider <vschneid@...hat.com>
> ---
> include/linux/sched/idle.h | 3 +--
> kernel/sched/fair.c | 27 ++++++++++++---------------
> 2 files changed, 13 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/sched/idle.h b/include/linux/sched/idle.h
> index 478084f9105e..4a6423700ffc 100644
> --- a/include/linux/sched/idle.h
> +++ b/include/linux/sched/idle.h
> @@ -5,8 +5,7 @@
> #include <linux/sched.h>
>
> enum cpu_idle_type {
> - CPU_IDLE,
> - CPU_NOT_IDLE,
Could be set CPU_NOT_IDLE = 0 to help keeping in mind that 0 means
cpu is not idle even if we don't use it ?
> + CPU_IDLE = 1,
> CPU_NEWLY_IDLE,
> CPU_MAX_IDLE_TYPES
> };
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 64ae3d8dc93b..f11fc6dd39b1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9070,7 +9070,7 @@ static int detach_tasks(struct lb_env *env)
> * We don't want to steal all, otherwise we may be treated likewise,
> * which could at worst lead to a livelock crash.
> */
> - if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
> + if (env->idle && env->src_rq->nr_running <= 1)
> break;
>
> env->loop++;
> @@ -9803,7 +9803,7 @@ static inline bool smt_vs_nonsmt_groups(struct sched_group *sg1,
> static inline bool smt_balance(struct lb_env *env, struct sg_lb_stats *sgs,
> struct sched_group *group)
> {
> - if (env->idle == CPU_NOT_IDLE)
> + if (!env->idle)
> return false;
>
> /*
> @@ -9827,7 +9827,7 @@ static inline long sibling_imbalance(struct lb_env *env,
> int ncores_busiest, ncores_local;
> long imbalance;
>
> - if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
> + if (!env->idle || !busiest->sum_nr_running)
> return 0;
>
> ncores_busiest = sds->busiest->cores;
> @@ -9927,8 +9927,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> sgs->group_misfit_task_load = rq->misfit_task_load;
> *sg_status |= SG_OVERLOAD;
> }
> - } else if ((env->idle != CPU_NOT_IDLE) &&
> - sched_reduced_capacity(rq, env->sd)) {
> + } else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
> /* Check for a task running on a CPU with reduced capacity */
> if (sgs->group_misfit_task_load < load)
> sgs->group_misfit_task_load = load;
> @@ -9940,7 +9939,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> sgs->group_weight = group->group_weight;
>
> /* Check if dst CPU is idle and preferred to this group */
> - if (!local_group && env->idle != CPU_NOT_IDLE && sgs->sum_h_nr_running &&
> + if (!local_group && env->idle && sgs->sum_h_nr_running &&
> sched_group_asym(env, sgs, group))
> sgs->group_asym_packing = 1;
>
> @@ -10698,7 +10697,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> * waiting task in this overloaded busiest group. Let's
> * try to pull it.
> */
> - if (env->idle != CPU_NOT_IDLE && env->imbalance == 0) {
> + if (env->idle && env->imbalance == 0) {
> env->migration_type = migrate_task;
> env->imbalance = 1;
> }
> @@ -10913,7 +10912,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> goto force_balance;
>
> if (busiest->group_type != group_overloaded) {
> - if (env->idle == CPU_NOT_IDLE) {
> + if (!env->idle) {
> /*
> * If the busiest group is not overloaded (and as a
> * result the local one too) but this CPU is already
> @@ -11121,7 +11120,7 @@ asym_active_balance(struct lb_env *env)
> * the lower priority @env::dst_cpu help it. Do not follow
> * CPU priority.
> */
> - return env->idle != CPU_NOT_IDLE && sched_use_asym_prio(env->sd, env->dst_cpu) &&
> + return env->idle && sched_use_asym_prio(env->sd, env->dst_cpu) &&
> (sched_asym_prefer(env->dst_cpu, env->src_cpu) ||
> !sched_use_asym_prio(env->sd, env->src_cpu));
> }
> @@ -11159,7 +11158,7 @@ static int need_active_balance(struct lb_env *env)
> * because of other sched_class or IRQs if more capacity stays
> * available on dst_cpu.
> */
> - if ((env->idle != CPU_NOT_IDLE) &&
> + if (env->idle &&
> (env->src_rq->cfs.h_nr_running == 1)) {
> if ((check_cpu_capacity(env->src_rq, sd)) &&
> (capacity_of(env->src_cpu)*sd->imbalance_pct < capacity_of(env->dst_cpu)*100))
> @@ -11735,8 +11734,8 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
> * env->dst_cpu, so we can't know our idle
> * state even if we migrated tasks. Update it.
> */
> - idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;
> - busy = idle != CPU_IDLE && !sched_idle_cpu(cpu);
> + idle = idle_cpu(cpu);
> + busy = !idle && !sched_idle_cpu(cpu);
> }
> sd->last_balance = jiffies;
> interval = get_sd_balance_interval(sd, busy);
> @@ -12416,9 +12415,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
> {
> struct rq *this_rq = this_rq();
> - enum cpu_idle_type idle = this_rq->idle_balance ?
> - CPU_IDLE : CPU_NOT_IDLE;
> -
> + enum cpu_idle_type idle = this_rq->idle_balance;
> /*
> * If this CPU has a pending nohz_balance_kick, then do the
> * balancing on behalf of the other idle CPUs whose ticks are
> --
> 2.40.1
>
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