Re: [PATCH v3 0/2] sched: Minor changes for rd->overload access
From: Shrikanth Hegde
Date: Thu Mar 28 2024 - 13:21:07 EST
On 3/28/24 4:37 PM, Ingo Molnar wrote:
>
> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>> Plus I've applied a patch to rename ::overload to ::overloaded. It is
>> silly to use an ambiguous noun instead of a clear adjective when naming
>> such a flag ...
>
> Plus SG_OVERLOAD should be SG_OVERLOADED as well - it now looks in line
> with SG_OVERUTILIZED:
>
> /* Scheduling group status flags */
> #define SG_OVERLOADED 0x1 /* More than one runnable task on a CPU. */
> #define SG_OVERUTILIZED 0x2 /* One or more CPUs are over-utilized. */
>
> My followup question is: why are these a bitmask, why not separate
> flags?
>
> AFAICS we only ever set them separately:
>
> thule:~/tip> git grep SG_OVER kernel/sched/
> kernel/sched/fair.c: set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
> kernel/sched/fair.c: *sg_status |= SG_OVERLOADED;
> kernel/sched/fair.c: *sg_status |= SG_OVERUTILIZED;
> kernel/sched/fair.c: *sg_status |= SG_OVERLOADED;
> kernel/sched/fair.c: set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
> kernel/sched/fair.c: sg_status & SG_OVERUTILIZED);
> kernel/sched/fair.c: } else if (sg_status & SG_OVERUTILIZED) {
> kernel/sched/fair.c: set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
> kernel/sched/sched.h:#define SG_OVERLOADED 0x1 /* More than one runnable task on a CPU. */
> kernel/sched/sched.h:#define SG_OVERUTILIZED 0x2 /* One or more CPUs are over-utilized. */
> kernel/sched/sched.h: set_rd_overloaded(rq->rd, SG_OVERLOADED);
>
> In fact this results in suboptimal code:
>
> /* update overload indicator if we are at root domain */
> set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
>
> /* Update over-utilization (tipping point, U >= 0) indicator */
> set_rd_overutilized_status(env->dst_rq->rd,
> sg_status & SG_OVERUTILIZED);
>
> Note how the bits that got mixed together in sg_status now have to be
> masked out individually.
>
> The sg_status bitmask appears to make no sense at all to me.
>
> By turning these into individual bool flags we could also do away with
> all the extra SG_OVERLOADED/SG_OVERUTILIZED abstraction.
>
> Ie. something like the patch below? Untested.
Looks good. I see it is merged to sched/core.
Did a boot with that patch and hackbench is showing same results 320 CPU system.
>
> Thanks,
>
> Ingo
>
> =================>
> From: Ingo Molnar <mingo@xxxxxxxxxx>
> Date: Thu, 28 Mar 2024 12:00:14 +0100
> Subject: [PATCH] sched/balancing: Simplify the sg_status bitmask and use separate ->overloaded and ->overutilized flags
>
> SG_OVERLOADED and SG_OVERUTILIZED flags plus the sg_status bitmask are an
> unnecessary complication that only make the code harder to read and slower.
>
> We only ever set them separately:
>
> thule:~/tip> git grep SG_OVER kernel/sched/
> kernel/sched/fair.c: set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
> kernel/sched/fair.c: *sg_status |= SG_OVERLOADED;
> kernel/sched/fair.c: *sg_status |= SG_OVERUTILIZED;
> kernel/sched/fair.c: *sg_status |= SG_OVERLOADED;
> kernel/sched/fair.c: set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
> kernel/sched/fair.c: sg_status & SG_OVERUTILIZED);
> kernel/sched/fair.c: } else if (sg_status & SG_OVERUTILIZED) {
> kernel/sched/fair.c: set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
> kernel/sched/sched.h:#define SG_OVERLOADED 0x1 /* More than one runnable task on a CPU. */
> kernel/sched/sched.h:#define SG_OVERUTILIZED 0x2 /* One or more CPUs are over-utilized. */
> kernel/sched/sched.h: set_rd_overloaded(rq->rd, SG_OVERLOADED);
>
> And use them separately, which results in suboptimal code:
>
> /* update overload indicator if we are at root domain */
> set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
>
> /* Update over-utilization (tipping point, U >= 0) indicator */
> set_rd_overutilized_status(env->dst_rq->rd,
>
> Introduce separate sg_overloaded and sg_overutilized flags in update_sd_lb_stats()
> and its lower level functions, and change all of them to 'bool'.
>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
> kernel/sched/fair.c | 33 ++++++++++++++++-----------------
> kernel/sched/sched.h | 17 ++++++-----------
> 2 files changed, 22 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f29efd5f19f6..ebc8d5f855de 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6688,19 +6688,18 @@ static inline bool cpu_overutilized(int cpu)
> /*
> * overutilized value make sense only if EAS is enabled
> */
> -static inline int is_rd_overutilized(struct root_domain *rd)
> +static inline bool is_rd_overutilized(struct root_domain *rd)
> {
> return !sched_energy_enabled() || READ_ONCE(rd->overutilized);
> }
>
> -static inline void set_rd_overutilized(struct root_domain *rd,
> - unsigned int status)
> +static inline void set_rd_overutilized(struct root_domain *rd, bool flag)
> {
> if (!sched_energy_enabled())
> return;
>
> - WRITE_ONCE(rd->overutilized, status);
> - trace_sched_overutilized_tp(rd, !!status);
> + WRITE_ONCE(rd->overutilized, flag);
> + trace_sched_overutilized_tp(rd, flag);
> }
>
> static inline void check_update_overutilized_status(struct rq *rq)
> @@ -6711,7 +6710,7 @@ static inline void check_update_overutilized_status(struct rq *rq)
> */
>
> if (!is_rd_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
> - set_rd_overutilized(rq->rd, SG_OVERUTILIZED);
> + set_rd_overutilized(rq->rd, 1);
> }
> #else
> static inline void check_update_overutilized_status(struct rq *rq) { }
> @@ -9940,7 +9939,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> struct sd_lb_stats *sds,
> struct sched_group *group,
> struct sg_lb_stats *sgs,
> - int *sg_status)
> + bool *sg_overloaded,
> + bool *sg_overutilized)
> {
> int i, nr_running, local_group;
>
> @@ -9961,10 +9961,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> sgs->sum_nr_running += nr_running;
>
> if (nr_running > 1)
> - *sg_status |= SG_OVERLOADED;
> + *sg_overloaded = 1;
>
> if (cpu_overutilized(i))
> - *sg_status |= SG_OVERUTILIZED;
> + *sg_overutilized = 1;
>
> #ifdef CONFIG_NUMA_BALANCING
> sgs->nr_numa_running += rq->nr_numa_running;
> @@ -9986,7 +9986,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> /* Check for a misfit task on the cpu */
> if (sgs->group_misfit_task_load < rq->misfit_task_load) {
> sgs->group_misfit_task_load = rq->misfit_task_load;
> - *sg_status |= SG_OVERLOADED;
> + *sg_overloaded = 1;
> }
> } else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
> /* Check for a task running on a CPU with reduced capacity */
> @@ -10612,7 +10612,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> struct sg_lb_stats *local = &sds->local_stat;
> struct sg_lb_stats tmp_sgs;
> unsigned long sum_util = 0;
> - int sg_status = 0;
> + bool sg_overloaded = 0, sg_overutilized = 0;
>
> do {
> struct sg_lb_stats *sgs = &tmp_sgs;
> @@ -10628,7 +10628,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> update_group_capacity(env->sd, env->dst_cpu);
> }
>
> - update_sg_lb_stats(env, sds, sg, sgs, &sg_status);
> + update_sg_lb_stats(env, sds, sg, sgs, &sg_overloaded, &sg_overutilized);
>
> if (!local_group && update_sd_pick_busiest(env, sds, sg, sgs)) {
> sds->busiest = sg;
> @@ -10657,13 +10657,12 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>
> if (!env->sd->parent) {
> /* update overload indicator if we are at root domain */
> - set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
> + set_rd_overloaded(env->dst_rq->rd, sg_overloaded);
>
> /* Update over-utilization (tipping point, U >= 0) indicator */
> - set_rd_overutilized(env->dst_rq->rd,
> - sg_status & SG_OVERUTILIZED);
> - } else if (sg_status & SG_OVERUTILIZED) {
> - set_rd_overutilized(env->dst_rq->rd, SG_OVERUTILIZED);
> + set_rd_overutilized(env->dst_rq->rd, sg_overloaded);
> + } else if (sg_overutilized) {
> + set_rd_overutilized(env->dst_rq->rd, sg_overutilized);
> }
>
> update_idle_cpu_scan(env, sum_util);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 07c6669b8250..7c39dbf31f75 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -713,7 +713,7 @@ struct rt_rq {
> } highest_prio;
> #endif
> #ifdef CONFIG_SMP
> - int overloaded;
> + bool overloaded;
> struct plist_head pushable_tasks;
>
> #endif /* CONFIG_SMP */
> @@ -757,7 +757,7 @@ struct dl_rq {
> u64 next;
> } earliest_dl;
>
> - int overloaded;
> + bool overloaded;
>
> /*
> * Tasks on this rq that can be pushed away. They are kept in
> @@ -850,10 +850,6 @@ struct perf_domain {
> struct rcu_head rcu;
> };
>
> -/* Scheduling group status flags */
> -#define SG_OVERLOADED 0x1 /* More than one runnable task on a CPU. */
> -#define SG_OVERUTILIZED 0x2 /* One or more CPUs are over-utilized. */
> -
> /*
> * We add the notion of a root-domain which will be used to define per-domain
> * variables. Each exclusive cpuset essentially defines an island domain by
> @@ -874,10 +870,10 @@ struct root_domain {
> * - More than one runnable task
> * - Running task is misfit
> */
> - int overloaded;
> + bool overloaded;
>
> /* Indicate one or more cpus over-utilized (tipping point) */
> - int overutilized;
> + bool overutilized;
>
> /*
> * The bit corresponding to a CPU gets set here if such CPU has more
> @@ -2540,9 +2536,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
> }
>
> #ifdef CONFIG_SMP
> - if (prev_nr < 2 && rq->nr_running >= 2) {
> - set_rd_overloaded(rq->rd, SG_OVERLOADED);
> - }
> + if (prev_nr < 2 && rq->nr_running >= 2)
> + set_rd_overloaded(rq->rd, 1);
> #endif
>
> sched_update_tick_dependency(rq);