linux-kernel - Re: [PATCH] sched/core: sched_getattr returning consistent sched

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Message-ID: <eeecc92b-4b9a-670e-646a-a711ac7bf814@arm.com>
Date:   Wed, 20 Dec 2017 12:32:47 +0000
From:   Alessio Balsini <alessio.balsini@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Luca Abeni <luca.abeni@...tannapisa.it>
Subject: Re: [PATCH] sched/core: sched_getattr returning consistent
 sched_priority

Hi,

On 19/12/17 16:16, Peter Zijlstra wrote:
[...]
>> This inaccuracy was introduced in 06a76fe08d4d, that moved the function
>> from core.c to deadline.c. Before that, it was making more sense to
>> access sched_priority, either if the function name __getparam_dl() was
>> misleading.
>
> OK, I'm dense, what?

Too verbose? :)
What about:

---8<---
sched/core: sched_getattr cleanup for sched_priority

Always initialise the sched_priority field of the sched_attr struct
returned by sched_getattr() with the rt_priority of the process.
This makes the code even more readable and saves a compare+jump.
The consistency of the rt_priority value is guaranteed by
__sched_setscheduler() that requires:
  - 1 <= sched_priority <= 99 for RT tasks, and
  - sched_priority == 0 otherwise.

Remove the sched_priority returned by __getparam_dl() since is not in the
scope of the DL class and does not affect the behaviour of the system.
---8<---

?

>
>> [...]
>> -    attr->sched_priority = p->rt_priority;
>>      attr->sched_runtime = dl_se->dl_runtime;
>>      attr->sched_deadline = dl_se->dl_deadline;
>>      attr->sched_period = dl_se->dl_period;
>
> That seems sane.

Good.

>>      attr.sched_policy = p->policy;
>>      if (p->sched_reset_on_fork)
>>              attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
>> +    attr.sched_priority = p->rt_priority;
>>      if (task_has_dl_policy(p))
>>              __getparam_dl(p, &attr);
>> -    else if (task_has_rt_policy(p))
>> -            attr.sched_priority = p->rt_priority;
>> -    else
>> +    else if (task_has_fair_policy(p))
>>              attr.sched_nice = task_nice(p);
>>
>>      rcu_read_unlock();
>
> This is confusing, why unconditionally assign? We initialize to 0, and
> if it must be 0 for !RT, then we should only assign when rt.
>

Right! But as I highlighted in the new changelog, this saves a
compare+jump and, since attr.sched_priority is in the stack, that data
assignment should not cause any relevant cache-related performance loss.

For the same reason, IMHO this cleanup could also be extended to the
sched_nice field:

---8<---
@@ -4606,12 +4606,10 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid,
struct sched_attr __user *, uattr,
         attr.sched_policy = p->policy;
         if (p->sched_reset_on_fork)
                 attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
+       attr.sched_nice = task_nice(p);
+       attr.sched_priority = p->rt_priority;
         if (task_has_dl_policy(p))
                 __getparam_dl(p, &attr);
-       else if (task_has_rt_policy(p))
-               attr.sched_priority = p->rt_priority;
-       else
-               attr.sched_nice = task_nice(p);

         rcu_read_unlock();
---8<---

This should make the code even more readable and causes no problem since
the documentation does not specify sched_nice field values for !FAIR
classes.
I left the task_has_dl_policy(p): __getparam_dl() represents a real
waste of time when !DL.

Thanks,
Alessio
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