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

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

Hi,

> On Jul 23, 2020, at 9:57 AM, Li, Aubrey <aubrey.li@linux.intel.com> wrote:
> 
> On 2020/7/22 22:32, benbjiang(蒋彪) wrote:
>> 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.
> Choosing target cpu could increase the runnable task number on the target runqueue, this
> could trigger busiest->nr_running > 1 logic and makes the idle sibling trying to pull but
> not success(due to cookie not match). Putting task to the idle sibling is relatively stable IMHO.

I’m afraid that *unsuccessful* pullings between smts would not result in unstableness, because
the load-balance always do periodicly , and unsuccess means nothing happen.
On the contrary, unmatched sibling tasks running concurrently could bring forced-idle to each other repeatedly,
Which is more unstable, and more costly when pick_next_task for all siblings.
In consideration of currently load-balance being not fully aware of core-scheduling, and can not improve
the *unmatched sibling* case, the *find_idlest_** entry should try its best to avoid the case, IMHO.
Also, just an advice and  an option. :)

Thx.
Regards,
Jiang  

> 
>> 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.
> I'm not sure if it's more cache friendly as the target is busy, and the coming task
> is a cookie unmatched task.
> 
>> 
>>> 
>>>> 
>>>>> +#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).
> 
> Though find_idlest_group_cpu() will check p->cpus_ptr again, I believe this is a good catch and
> should be fixed in the next iteration.
> 
> Thanks,
> -Aubrey