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Date: Fri, 1 Mar 2024 19:44:34 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org
Cc: yu.c.chen@...el.com, dietmar.eggemann@....com,
linux-kernel@...r.kernel.org, nysal@...ux.ibm.com,
aboorvad@...ux.ibm.com, srikar@...ux.vnet.ibm.com, vschneid@...hat.com,
pierre.gondois@....com, morten.rasmussen@....com, qyousef@...alina.io
Subject: Re: [PATCH v3 1/2] sched/fair: Add EAS checks before updating
overutilized
On 2/29/24 4:10 PM, Shrikanth Hegde wrote:
> Overutilized field of root domain is only used for EAS(energy aware scheduler)
> to decide whether to do load balance or not. It is not used if EAS
> not possible.
>
> Currently enqueue_task_fair and task_tick_fair accesses, sometime updates
> this field. In update_sd_lb_stats it is updated often. This causes cache
> contention due to true sharing and burns a lot of cycles. overload and
> overutilized are part of the same cacheline. Updating it often invalidates
> the cacheline. That causes access to overload to slow down due to
> false sharing. Hence add EAS check before accessing/updating this field.
> EAS check is optimized at compile time or it is a static branch.
> Hence it shouldn't cost much.
>
> With the patch, both enqueue_task_fair and newidle_balance don't show
> up as hot routines in perf profile.
>
> 6.8-rc4:
> 7.18% swapper [kernel.vmlinux] [k] enqueue_task_fair
> 6.78% s [kernel.vmlinux] [k] newidle_balance
> +patch:
> 0.14% swapper [kernel.vmlinux] [k] enqueue_task_fair
> 0.00% swapper [kernel.vmlinux] [k] newidle_balance
>
> Minor change: trace_sched_overutilized_tp expect that second argument to
> be bool. So do a int to bool conversion for that.
>
> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
> kernel/sched/fair.c | 47 ++++++++++++++++++++++++++++-----------------
> 1 file changed, 29 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6a16129f9a5c..1f7d62b7c26f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6670,15 +6670,29 @@ static inline bool cpu_overutilized(int cpu)
> return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
> }
>
> -static inline void update_overutilized_status(struct rq *rq)
> +static inline void set_rd_overutilized_status(struct root_domain *rd,
> + unsigned int status)
> {
> - if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu)) {
> - WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED);
> - trace_sched_overutilized_tp(rq->rd, SG_OVERUTILIZED);
> - }
> + WRITE_ONCE(rd->overutilized, status);
> + trace_sched_overutilized_tp(rd, !!status);
> +}
> +
> +static inline void check_update_overutilized_status(struct rq *rq)
> +{
> + /*
> + * overutilized field is used for load balancing decisions only
> + * if energy aware scheduler is being used
> + */
> + if (!sched_energy_enabled())
> + return;
> +
> + if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu))
> + set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
> }
> #else
> -static inline void update_overutilized_status(struct rq *rq) { }
> +static inline void check_update_overutilized_status(struct rq *rq) { }
> +static inline void set_rd_overutilized_status(struct root_domain *rd,
> + unsigned int status) { }
> #endif
>
> /* Runqueue only has SCHED_IDLE tasks enqueued */
> @@ -6779,7 +6793,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> * and the following generally works well enough in practice.
> */
> if (!task_new)
> - update_overutilized_status(rq);
> + check_update_overutilized_status(rq);
>
> enqueue_throttle:
> assert_list_leaf_cfs_rq(rq);
> @@ -9902,7 +9916,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> if (nr_running > 1)
> *sg_status |= SG_OVERLOAD;
>
> - if (cpu_overutilized(i))
> + if (sched_energy_enabled() && cpu_overutilized(i))
> *sg_status |= SG_OVERUTILIZED;
>
> #ifdef CONFIG_NUMA_BALANCING
> @@ -10596,19 +10610,16 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>
> if (!env->sd->parent) {
> - struct root_domain *rd = env->dst_rq->rd;
> -
> /* update overload indicator if we are at root domain */
> - WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);
> + WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);
>
> /* Update over-utilization (tipping point, U >= 0) indicator */
> - WRITE_ONCE(rd->overutilized, sg_status & SG_OVERUTILIZED);
> - trace_sched_overutilized_tp(rd, sg_status & SG_OVERUTILIZED);
> + if (sched_energy_enabled()) {
> + set_rd_overutilized_status(env->dst_rq->rd,
> + sg_status & SG_OVERUTILIZED);
> + }
> } else if (sg_status & SG_OVERUTILIZED) {
Sorry. I missed adding a check here.
Let me do that send out next version.
> - struct root_domain *rd = env->dst_rq->rd;
> -
> - WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
> - trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
> + set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
> }
>
> update_idle_cpu_scan(env, sum_util);
> @@ -12609,7 +12620,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
> task_tick_numa(rq, curr);
>
> update_misfit_status(curr, rq);
> - update_overutilized_status(task_rq(curr));
> + check_update_overutilized_status(task_rq(curr));
>
> task_tick_core(rq, curr);
> }
> --
> 2.39.3
>
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