Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Nov 17, 2020 at 06:19:44PM -0500, Joel Fernandes (Google) wrote:
> From: Aubrey Li <aubrey.li@intel.com>
> 
>  - 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
> 
> Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Vineeth Remanan Pillai <viremana@linux.microsoft.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/fair.c  | 64 ++++++++++++++++++++++++++++++++++++++++----
>  kernel/sched/sched.h | 29 ++++++++++++++++++++
>  2 files changed, 88 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index de82f88ba98c..ceb3906c9a8a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1921,6 +1921,15 @@ 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
> +

Any reason this is under an #ifdef? In sched_core_cookie_match() won't
the check for sched_core_enabled() do the right thing even when
CONFIG_SCHED_CORE is not enabed?

>  		env->dst_cpu = cpu;
>  		if (task_numa_compare(env, taskimp, groupimp, maymove))
>  			break;
> @@ -5867,11 +5876,17 @@ 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 (sched_idle_cpu(i))
>  			return i;
>  
>  		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) {
>  				/*
> @@ -6129,8 +6144,18 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	for_each_cpu_wrap(cpu, cpus, target) {
>  		if (!--nr)
>  			return -1;
> -		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> -			break;
> +
> +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) {
> +#ifdef CONFIG_SCHED_CORE
> +			/*
> +			 * If Core Scheduling is enabled, select this cpu
> +			 * only if the process cookie matches core cookie.
> +			 */
> +			if (sched_core_enabled(cpu_rq(cpu)) &&
> +			    p->core_cookie == cpu_rq(cpu)->core->core_cookie)
> +#endif
> +				break;
> +		}
>  	}
>  
>  	time = cpu_clock(this) - time;
> @@ -7530,8 +7555,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  	 * We do not migrate tasks that are:
>  	 * 1) throttled_lb_pair, or
>  	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
> -	 * 3) running (obviously), or
> -	 * 4) are cache-hot on their current CPU.
> +	 * 3) task's cookie does not match with this CPU's core cookie
> +	 * 4) running (obviously), or
> +	 * 5) are cache-hot on their current CPU.
>  	 */
>  	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>  		return 0;
> @@ -7566,6 +7592,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  		return 0;
>  	}
>  
> +#ifdef CONFIG_SCHED_CORE
> +	/*
> +	 * Don't migrate task if the task's cookie does not match
> +	 * with the destination CPU's core cookie.
> +	 */
> +	if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
> +		return 0;
> +#endif
> +
>  	/* Record that we found atleast one task that could run on dst_cpu */
>  	env->flags &= ~LBF_ALL_PINNED;
>  
> @@ -8792,6 +8827,25 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>  					p->cpus_ptr))
>  			continue;
>  
> +#ifdef CONFIG_SCHED_CORE
> +		if (sched_core_enabled(cpu_rq(this_cpu))) {
> +			int i = 0;
> +			bool cookie_match = false;
> +
> +			for_each_cpu(i, sched_group_span(group)) {
> +				struct rq *rq = cpu_rq(i);
> +
> +				if (sched_core_cookie_match(rq, p)) {
> +					cookie_match = true;
> +					break;
> +				}
> +			}
> +			/* Skip over this group if no cookie matched */
> +			if (!cookie_match)
> +				continue;
> +		}
> +#endif
> +
>  		local_group = cpumask_test_cpu(this_cpu,
>  					       sched_group_span(group));
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e72942a9ee11..de553d39aa40 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1135,6 +1135,35 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
>  
>  bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
>  
> +/*
> + * Helper to check if the CPU's core cookie matches with the task's cookie
> + * when core scheduling is enabled.
> + * A special case is that the task's cookie always matches with CPU's core
> + * cookie if the CPU is in an idle core.
> + */
> +static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
> +{
> +	bool idle_core = true;
> +	int cpu;
> +
> +	/* Ignore cookie match if core scheduler is not enabled on the CPU. */
> +	if (!sched_core_enabled(rq))
> +		return true;
> +
> +	for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
> +		if (!available_idle_cpu(cpu)) {

I was looking at this snippet and comparing this to is_core_idle(), the
major difference is the check for vcpu_is_preempted(). Do we want to
call the core as non idle if any vcpu was preempted on this CPU?

> +			idle_core = false;
> +			break;
> +		}
> +	}
> +
> +	/*
> +	 * A CPU in an idle core is always the best choice for tasks with
> +	 * cookies.
> +	 */
> +	return idle_core || rq->core->core_cookie == p->core_cookie;
> +}
> +

Balbir Singh.