Re: [PATCH v5 3/4] sched: Allow sched_{get,set}attr to change latency_nice of the task

[Parth Shah <parth@linux.ibm.com>]

On 5/11/20 4:43 PM, Dietmar Eggemann wrote:
> On 28/02/2020 10:07, Parth Shah wrote:
>> Introduce the latency_nice attribute to sched_attr and provide a
>> mechanism to change the value with the use of sched_setattr/sched_getattr
>> syscall.
>>
>> Also add new flag "SCHED_FLAG_LATENCY_NICE" to hint the change in
>> latency_nice of the task on every sched_setattr syscall.
>>
>> Signed-off-by: Parth Shah <parth@linux.ibm.com>
>> Reviewed-by: Qais Yousef <qais.yousef@arm.com>
> 
> [...]
> 
> ndif /* _UAPI_LINUX_SCHED_TYPES_H */
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 866ea3d2d284..cd1fb9c8be26 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -4710,6 +4710,9 @@ static void __setscheduler_params(struct task_struct *p,
>>  	p->rt_priority = attr->sched_priority;
>>  	p->normal_prio = normal_prio(p);
>>  	set_load_weight(p, true);
>> +
>> +	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
>> +		p->latency_nice = attr->sched_latency_nice;
>>  }
> 
> How do you make sure that p->latency_nice can be set independently from
> p->static_prio?
> 
> AFAICS, util_clamp achieves this by relying on SCHED_FLAG_KEEP_PARAMS,
> so completely bypassing __setscheduler_params() and using it's own
> __setscheduler_uclamp().
> 

Right. good catch.
Use of SCHED_FLAG_LATENCY_NICE/SCHED_FLAG_ALL is must to change
latency_nice value, but currently setting latency_nice value also changes
static_prio.

One possible solution here is to move the above code to _setscheduler():

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6031ec58c7ae..44bcbf060718 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4731,9 +4731,6 @@ static void __setscheduler_params(struct task_struct *p,
        p->rt_priority = attr->sched_priority;
        p->normal_prio = normal_prio(p);
        set_load_weight(p, true);
-
-       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
-               p->latency_nice = attr->sched_latency_nice;
 }

 /* Actually do priority change: must hold pi & rq lock. */
@@ -4749,6 +4746,13 @@ static void __setscheduler(struct rq *rq, struct
task_struct *p,

        __setscheduler_params(p, attr);

+       /*
+        * Change latency_nice value only when SCHED_FLAG_LATENCY_NICE or
+        * SCHED_FLAG_ALL sched_flag is set.
+        */
+       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
+               p->latency_nice = attr->sched_latency_nice;
+

This should allow setting value only on above flags, also restricts setting
the value when SCHED_FLAG_KEEP_PARAMS/SCHED_FLAG_KEEP_ALL is passed.


Thanks,
Parth