linux-kernel - Re: [PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd

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Date:   Thu, 19 Mar 2020 11:46:11 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v2 9/9] sched/topology: Define and use shortcut pointers
 for wakeup sd_flag scan

On 11.03.20 19:16, Valentin Schneider wrote:
> Reworking select_task_rq_fair()'s domain walk exposed that !want_affine
> wakeups only look for highest sched_domain with the required sd_flag
> set. This is something we can cache at sched domain build time to slightly
> optimize select_task_rq_fair(). Note that this isn't a "free" optimization:
> it costs us 3 pointers per CPU.
> 
> Add cached per-CPU pointers for the highest domains with SD_BALANCE_WAKE,
> SD_BALANCE_EXEC and SD_BALANCE_FORK. Use them in select_task_rq_fair().
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c     | 25 +++++++++++++------------
>  kernel/sched/sched.h    |  3 +++
>  kernel/sched/topology.c | 12 ++++++++++++
>  3 files changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a6fca6817e92..40fb97062157 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6595,17 +6595,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	int want_affine = 0;
>  	int sd_flag;
>  
> -	switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
> -	case WF_TTWU:
> -		sd_flag = SD_BALANCE_WAKE;
> -		break;
> -	case WF_FORK:
> -		sd_flag = SD_BALANCE_FORK;
> -		break;
> -	default:
> -		sd_flag = SD_BALANCE_EXEC;
> -	}
> -
>  	if (wake_flags & WF_TTWU) {
>  		record_wakee(p);
>  
> @@ -6621,7 +6610,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  
>  	rcu_read_lock();
>  
> -	sd = highest_flag_domain(cpu, sd_flag);
> +	switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
> +	case WF_TTWU:
> +		sd_flag = SD_BALANCE_WAKE;
> +		sd = rcu_dereference(per_cpu(sd_balance_wake, cpu));

IMHO, since we hard-code 0*SD_BALANCE_WAKE in sd_init(), sd would always
be NULL, so !want_affine (i.e. wake_wide()) would still go sis().

SD_BALANCE_WAKE is no a topology related sd_flag so it can't be set from
outside. Since the sd->flags sysctl is now read-only, wouldn't this case
be redundant?

[...]