Re: [RFC PATCH 11/13] sched: migration changes for core scheduling

[Joel Fernandes]

On Wed, Mar 04, 2020 at 05:00:01PM +0000, vpillai wrote:
> From: Aubrey Li <aubrey.li@xxxxxxxxx>
> 
>  - Don't migrate if there is a cookie mismatch
>      Load balance tries to move task from busiest CPU to the
>      destination CPU. When core scheduling is enabled, if the
>      task's cookie does not match with the destination CPU's
>      core cookie, this task will be skipped by this CPU. This
>      mitigates the forced idle time on the destination CPU.
> 
>  - Select cookie matched idle CPU
>      In the fast path of task wakeup, select the first cookie matched
>      idle CPU instead of the first idle CPU.
> 
>  - Find cookie matched idlest CPU
>      In the slow path of task wakeup, find the idlest CPU whose core
>      cookie matches with task's cookie
> 
>  - Don't migrate task if cookie not match
>      For the NUMA load balance, don't migrate task to the CPU whose
>      core cookie does not match with task's cookie
> 
> Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Vineeth Remanan Pillai <vpillai@xxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c  | 55 +++++++++++++++++++++++++++++++++++++++++---
>  kernel/sched/sched.h | 29 +++++++++++++++++++++++
>  2 files changed, 81 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1c9a80d8dbb8..f42ceecb749f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1789,6 +1789,15 @@ static void task_numa_find_cpu(struct task_numa_env *env,
>  		if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
>  			continue;
>  
> +#ifdef CONFIG_SCHED_CORE
> +		/*
> +		 * Skip this cpu if source task's cookie does not match
> +		 * with CPU's core cookie.
> +		 */
> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> +			continue;
> +#endif
> +
>  		env->dst_cpu = cpu;
>  		task_numa_compare(env, taskimp, groupimp, maymove);
>  	}
> @@ -5660,8 +5669,13 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
>  
>  	/* Traverse only the allowed CPUs */
>  	for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
> +		struct rq *rq = cpu_rq(i);
> +
> +#ifdef CONFIG_SCHED_CORE
> +		if (!sched_core_cookie_match(rq, p))
> +			continue;
> +#endif
>  		if (available_idle_cpu(i)) {
> -			struct rq *rq = cpu_rq(i);
>  			struct cpuidle_state *idle = idle_get_state(rq);
>  			if (idle && idle->exit_latency < min_exit_latency) {
>  				/*
> @@ -5927,8 +5941,14 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  			return si_cpu;
>  		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>  			continue;
> +#ifdef CONFIG_SCHED_CORE
> +		if (available_idle_cpu(cpu) &&
> +		    sched_core_cookie_match(cpu_rq(cpu), p))
> +			break;
> +#else

select_idle_cpu() is called only if no idle core could be found in the LLC by
select_idle_core().

So, would it be better here to just do the cookie equality check directly
instead of calling the sched_core_cookie_match() helper?  More so, because
select_idle_sibling() is a fastpath.

AFAIR, that's what v4 did:

                if (available_idle_cpu(cpu))
#ifdef CONFIG_SCHED_CORE
                        if (sched_core_enabled(cpu_rq(cpu)) &&
                            (p->core_cookie == cpu_rq(cpu)->core->core_cookie))
                                break;
#else
                        break;
#endif


Thoughts? thanks,

 - Joel