Re: [RFC PATCH 11/16] sched: migration changes for core scheduling(Internet mail)

["benbjiang(蒋彪)" <benbjiang@tencent.com>]

Hi,

> On Jul 22, 2020, at 8:13 PM, Li, Aubrey <aubrey.li@linux.intel.com> wrote:
> 
> On 2020/7/22 16:54, benbjiang(蒋彪) wrote:
>> Hi, Aubrey,
>> 
>>> On Jul 1, 2020, at 5:32 AM, Vineeth Remanan Pillai <vpillai@digitalocean.com> 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
>>> 
>>> 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 <vpillai@digitalocean.com>
>>> ---
>>> 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 d16939766361..33dc4bf01817 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -2051,6 +2051,15 @@ 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;
>>> 		if (task_numa_compare(env, taskimp, groupimp, maymove))
>>> 			break;
>>> @@ -5963,11 +5972,17 @@ 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) {
>>> 				/*
>>> @@ -6224,8 +6239,18 @@ 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)
>> Why not also add similar logic in select_idle_smt to reduce forced-idle? :)
> We hit select_idle_smt after we scaned the entire LLC domain for idle cores
> and idle cpus and failed,so IMHO, an idle smt is probably a good choice under
> this scenario.

AFAIC, selecting idle sibling with unmatched cookie will cause unnecessary fored-idle, unfairness and latency, compared to choosing *target* cpu.
Besides, choosing *target* cpu may be more cache friendly. So IMHO, *target* cpu may be a better choice if cookie not match, instead of idle sibling.

> 
>> 
>>> +#endif
>>> +				break;
>>> +		}
>>> 	}
>>> 
>>> 	time = cpu_clock(this) - time;
>>> @@ -7609,8 +7634,9 @@ 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;
>>> @@ -7645,6 +7671,15 @@ 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;
>>> 
>>> @@ -8857,6 +8892,25 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>> 					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)) {
>> Should we consider the p->cpus_ptr here? like,
>> 			for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr ) {
> 
> This is already considered just above #ifdef CONFIG_SCHED_CORE, but not included
> in the patch file.
> 
> Thanks,
> -Aubrey

The above consideration is,
 8893                 /* Skip over this group if it has no CPUs allowed */
 8894                 if (!cpumask_intersects(sched_group_span(group),
 8895                                         p->cpus_ptr))
 8896                         continue;
 8897
It only considers the case of *p is not allowed for the whole group*, which is not enough.
If( cpumask_subset(p->cpus_ptr, sched_group_span(group)), the following sched_core_cookie_match() may choose a *wrong(not allowed)* cpu to match cookie. In that case, the matching result could be confusing and lead to wrong result.
On the other hand, considering p->cpus_ptr here could reduce the loop times and cost, if cpumask_and(p->cpus_ptr, sched_group_span(group)) is the subset of sched_group_span(group).

Thx.
Regards,
Jiang

> 
>> 				...
>> 			}
>> Thx.
>> Regards,
>> Jiang
>> 
>>> +				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 464559676fd2..875796d43fca 100644
>>> --- a/kernel/sched/sched.h
>>> +++ b/kernel/sched/sched.h
>>> @@ -1089,6 +1089,35 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
>>> bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
>>> void sched_core_adjust_sibling_vruntime(int cpu, bool coresched_enabled);
>>> 
>>> +/*
>>> + * 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)) {
>>> +			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;
>>> +}
>>> +
>>> extern void queue_core_balance(struct rq *rq);
>>> 
>>> #else /* !CONFIG_SCHED_CORE */
>>> -- 
>>> 2.17.1