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

["Li, Aubrey" <aubrey.li@linux.intel.com>]

On 2020/11/26 16:32, Balbir Singh wrote:
> On Thu, Nov 26, 2020 at 11:20:41AM +0800, Li, Aubrey wrote:
>> On 2020/11/26 6:57, Balbir Singh wrote:
>>> On Wed, Nov 25, 2020 at 11:12:53AM +0800, Li, Aubrey wrote:
>>>> On 2020/11/24 23:42, Peter Zijlstra wrote:
>>>>> On Mon, Nov 23, 2020 at 12:36:10PM +0800, Li, Aubrey wrote:
>>>>>>>> +#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?> 
>>>>>> Yes, sched_core_enabled works properly when CONFIG_SCHED_CORE is not
>>>>>> enabled. But when CONFIG_SCHED_CORE is not enabled, it does not make
>>>>>> sense to leave a core scheduler specific function here even at compile
>>>>>> time. Also, for the cases in hot path, this saves CPU cycles to avoid
>>>>>> a judgment.
>>>>>
>>>>> No, that's nonsense. If it works, remove the #ifdef. Less (#ifdef) is
>>>>> more.
>>>>>
>>>>
>>>> Okay, I pasted the refined patch here.
>>>> @Joel, please let me know if you want me to send it in a separated thread.
>>>>
>>>
>>> You still have a bunch of #ifdefs, can't we just do
>>>
>>> #ifndef CONFIG_SCHED_CORE
>>> static inline bool sched_core_enabled(struct rq *rq)
>>> {
>>>         return false;
>>> }
>>> #endif
>>>
>>> and frankly I think even that is not needed because there is a jump
>>> label __sched_core_enabled that tells us if sched_core is enabled or
>>> not.
>>
>> Hmm..., I need another wrapper for CONFIG_SCHED_CORE specific variables.
>> How about this one?
>>
> 
> Much better :)
>  
>> Thanks,
>> -Aubrey
>>
>> From 61dac9067e66b5b9ea26c684c8c8235714bab38a Mon Sep 17 00:00:00 2001
>> From: Aubrey Li <aubrey.li@linux.intel.com>
>> Date: Thu, 26 Nov 2020 03:08:04 +0000
>> Subject: [PATCH] sched: migration changes for core scheduling
>>
>>  - 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  | 57 ++++++++++++++++++++++++++++++++++++++++----
>>  kernel/sched/sched.h | 43 +++++++++++++++++++++++++++++++++
>>  2 files changed, 95 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index de82f88ba98c..70dd013dff1d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1921,6 +1921,13 @@ static void task_numa_find_cpu(struct task_numa_env *env,
>>  		if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
>>  			continue;
>>  
>> +		/*
>> +		 * 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;
>> +
>>  		env->dst_cpu = cpu;
>>  		if (task_numa_compare(env, taskimp, groupimp, maymove))
>>  			break;
>> @@ -5867,11 +5874,15 @@ 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);
>> +
>> +		if (!sched_core_cookie_match(rq, p))
>> +			continue;
>> +
>>  		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 +6140,19 @@ 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)) {
>> +			/*
>> +			 * If Core Scheduling is enabled, select this cpu
>> +			 * only if the process cookie matches core cookie.
>> +			 */
>> +			if (sched_core_enabled(cpu_rq(cpu))) {
>> +				if (__cookie_match(cpu_rq(cpu), p))
>> +					break;
>> +			} else {
>> +				break;
>> +			}
>> +		}
> 
> Isn't this better and equivalent?
> 
> 	if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
> 		sched_core_cookie_match(cpu_rq(cpu), p))
> 		break;
>

 
That's my previous implementation in the earlier version.
But since here is the hot code path, we want to remove the idle
core check in sched_core_cookie_match.

>>  	}
>>  
>>  	time = cpu_clock(this) - time;
>> @@ -7530,8 +7552,9 @@ 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 +7589,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>  		return 0;
>>  	}
>>  
>> +	/*
>> +	 * 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;
>> +
>>  	/* Record that we found atleast one task that could run on dst_cpu */
>>  	env->flags &= ~LBF_ALL_PINNED;
>>  
>> @@ -8792,6 +8822,23 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>>  					p->cpus_ptr))
>>  			continue;
>>  
>> +		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;
>> +		}
>> +
> 
> Again, I think this can be refactored because sched_core_cookie_match checks
> for sched_core_enabled()
> 
> 	int i = 0;
> 	bool cookie_match = false;
> 	for_each_cpu(i, sched_group_span(group)) {
> 		if (sched_core_cookie_match(cpu_rq(i), p))
> 			break;
> 	}
> 	if (i >= nr_cpu_ids)
> 		continue;

There is a loop here when CONFIG_SCHED_CORE=n, which is unwanted I guess.

Thanks,
-Aubrey