Re: [RFC PATCH 1/2] sched: Rate limit migrations to 1 per 2ms per task

[Tim Chen]

On Tue, 2023-09-05 at 13:11 -0400, Mathieu Desnoyers wrote:
> Rate limit migrations to 1 migration per 2 milliseconds per task. On a
> kernel with EEVDF scheduler (commit b97d64c722598ffed42ece814a2cb791336c6679),
> this speeds up hackbench from 62s to 45s on AMD EPYC 192-core (over 2 sockets).
> 
> 
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 479db611f46e..0d294fce261d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4510,6 +4510,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>  	p->se.vruntime			= 0;
>  	p->se.vlag			= 0;
>  	p->se.slice			= sysctl_sched_base_slice;
> +	p->se.next_migration_time	= 0;

It seems like the next_migration_time should be initialized to the current time,
in case the system run for a long time and clock wrap around could cause problem.

>  	INIT_LIST_HEAD(&p->se.group_node);
>  
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d92da2d78774..24ac69913005 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -960,6 +960,14 @@ int sched_update_scaling(void)
>  
>  static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se);
>  
> +static bool should_migrate_task(struct task_struct *p, int prev_cpu)
> +{
> +	/* Rate limit task migration. */
> +	if (sched_clock_cpu(prev_cpu) < p->se.next_migration_time)

Should we use time_before(sched_clock_cpu(prev_cpu), p->se.next_migration_time) ?

> +	       return false;
> +	return true;
> +}
> +

Thanks.

Tim