Re: [PATCH v4 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 19/07/2021 18:16, Quentin Perret wrote:
> SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that
> the call must not touch scheduling parameters (nice or priority). This

What about DL params (runtime, deadline, period)?

Uclamp is not for DL but we could (*) still set uclamp values on a DL
task. Obviously they would only be used when the task switches policy.

On tip/sched/core:

root@juno:~# chrt -d -P 1000000000 -T 100000000 -p 0 1671

root@juno:~# uclampset -m200 -M400 -p 1671

root@juno:~# cat /proc/1671/sched | grep uclamp
uclamp.min                                   :                  200
uclamp.max                                   :                  400
effective uclamp.min                         :                  200
effective uclamp.max                         :                  400

root@juno:~# chrt -o -p 0
pid 1702's current scheduling policy: SCHED_OTHER
pid 1702's current scheduling priority: 0

root@juno:~# cat /proc/1671/sched | grep uclamp
uclamp.min                                   :                  200
uclamp.max                                   :                  400
effective uclamp.min                         :                  200
effective uclamp.max                         :                  400


> is particularly handy for uclamp when used in conjunction with
> SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only
> impacts uclamp values.
> 
> However, sched_setattr always checks whether the priorities and nice
> values passed in sched_attr are valid first, even if those never get

+ DL params (__checkparam_dl())

> used down the line. This is useless at best since userspace can
> trivially bypass this check to set the uclamp values by specifying low
> priorities. However, it is cumbersome to do so as there is no single
> expression of this that skips both RT and CFS checks at once. As such,
> userspace needs to query the task policy first with e.g. sched_getattr
> and then set sched_attr.sched_priority accordingly. This is racy and
> slower than a single call.
> 
> As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS
> is specified, simply inherit them in this case to match the policy
> inheritance of SCHED_FLAG_KEEP_POLICY.
> 
> Reported-by: Wei Wang <wvw@google.com>
> Reviewed-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  kernel/sched/core.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e801d2c3077b..914076eab242 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7332,6 +7332,16 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
>  	return -E2BIG;
>  }
>  
> +static void get_params(struct task_struct *p, struct sched_attr *attr)
> +{
> +	if (task_has_dl_policy(p))
> +		__getparam_dl(p, attr);

(*) This changes the behaviour when setting uclamp values on a DL task.

Before uclamp values could be set but now, because of

  void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
    ..
    attr->sched_flags = dl_se->flags

SCHED_FLAG_UTIL_CLAMP gets overwritten and  __sched_setscheduler() bails in:

    if (unlikely(policy == p->policy)) {
      ...
      retval = 0;
      goto unlock;
    }
  change:

I.e. the:

      if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
        goto change;

can't trigger anymore.


> +	else if (task_has_rt_policy(p))
> +		attr->sched_priority = p->rt_priority;
> +	else
> +		attr->sched_nice = task_nice(p);
> +}
> +
>  /**
>   * sys_sched_setscheduler - set/change the scheduler policy and RT priority
>   * @pid: the pid in question.
> @@ -7393,6 +7403,8 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
>  	rcu_read_unlock();
>  
>  	if (likely(p)) {
> +		if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS)
> +			get_params(p, &attr);

SCHED_FLAG_KEEP_PARAMS is handled here but SCHED_FLAG_KEEP_POLICY
outside (before) the `if (likely(p))`?

>  		retval = sched_setattr(p, &attr);
>  		put_task_struct(p);
>  	}

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