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Subject: Re: [RFC PATCH 11/16] sched: migration changes for core
 scheduling(Internet mail)
To:     =?UTF-8?B?YmVuYmppYW5nKOiSi+W9qik=?= <benbjiang@tencent.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        Aubrey Li <aubrey.li@intel.com>,
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        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
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From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <6ab8a001-ae5e-e484-c571-90d6931004e7@linux.intel.com>
Date:   Thu, 23 Jul 2020 11:35:56 +0800
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On 2020/7/23 10:42, benbjiang(蒋彪) wrote:
> 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.
unsuccess pulling means more unnecessary overhead in load balance.

> 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.
Not worse than two tasks ping-pong on the same target run queue I guess, and better if
- task1(cookie A) is running on the target, and task2(cookie B) in the runqueue,
- task3(cookie B) coming

If task3 chooses target's sibling, it could have a chance to run concurrently with task2.
But if task3 chooses target, it will wait for next pulling luck of load balancer

Thanks,
-Aubrey

> 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.
>>