Re: [PATCH v5 1/3] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity()

[Waiman Long <longman@xxxxxxxxxx>]

On 8/17/22 04:41, Peter Zijlstra wrote:
> On Tue, Aug 16, 2022 at 03:27:32PM -0400, Waiman Long wrote:
> > @@ -2981,25 +2969,21 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
> >   		goto err_unlock;
> >   	}
> >   
> > -	if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) {
> > +
> > +	if (p->user_cpus_ptr)
> > +		not_empty = cpumask_and(new_mask, p->user_cpus_ptr, subset_mask);
> > +	else
> > +		not_empty = cpumask_and(new_mask, cpu_online_mask, subset_mask);
> > +
> > +	if (!not_empty) {
> >   		err = -EINVAL;
> >   		goto err_unlock;
> >   	}
> >   
> > -	/*
> > -	 * We're about to butcher the task affinity, so keep track of what
> > -	 * the user asked for in case we're able to restore it later on.
> > -	 */
> > -	if (user_mask) {
> > -		cpumask_copy(user_mask, p->cpus_ptr);
> > -		p->user_cpus_ptr = user_mask;
> > -	}
> > -
> >   	return __set_cpus_allowed_ptr_locked(p, new_mask, 0, rq, &rf);
> >   
> >   err_unlock:
> >   	task_rq_unlock(rq, p, &rf);
> > -	kfree(user_mask);
> >   	return err;
> >   }
> >   
> > @@ -3049,34 +3033,27 @@ void force_compatible_cpus_allowed_ptr(struct task_struct *p)
> >   }
> >   
> >   static int
> > -__sched_setaffinity(struct task_struct *p, const struct cpumask *mask);
> > +__sched_setaffinity(struct task_struct *p, const struct cpumask *mask, bool save_mask);
> >   
> >   /*
> >    * Restore the affinity of a task @p which was previously restricted by a
> > - * call to force_compatible_cpus_allowed_ptr(). This will clear (and free)
> > - * @p->user_cpus_ptr.
> > + * call to force_compatible_cpus_allowed_ptr().
> >    *
> >    * It is the caller's responsibility to serialise this with any calls to
> >    * force_compatible_cpus_allowed_ptr(@p).
> >    */
> >   void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> >   {
> > -	struct cpumask *user_mask = p->user_cpus_ptr;
> > -	unsigned long flags;
> > +	const struct cpumask *user_mask = p->user_cpus_ptr;
> > +
> > +	if (!user_mask)
> > +		user_mask = cpu_online_mask;
> >   
> >   	/*
> > -	 * Try to restore the old affinity mask. If this fails, then
> > -	 * we free the mask explicitly to avoid it being inherited across
> > -	 * a subsequent fork().
> > +	 * Try to restore the old affinity mask with __sched_setaffinity().
> > +	 * Cpuset masking will be done there too.
> >   	 */
> > -	if (!user_mask || !__sched_setaffinity(p, user_mask))
> > -		return;
> > -
> > -	raw_spin_lock_irqsave(&p->pi_lock, flags);
> > -	user_mask = clear_user_cpus_ptr(p);
> > -	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> > -
> > -	kfree(user_mask);
> > +	__sched_setaffinity(p, user_mask, false);
> >   }
> >   
> >   void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>
> Would it not be simpler to write it something like so?
>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 03053eebb22e..cdae4d50a588 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2955,7 +2955,6 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
>   	struct rq_flags rf;
>   	struct rq *rq;
>   	int err;
> -	bool not_empty;
>   
>   	rq = task_rq_lock(p, &rf);
>   
> @@ -2969,13 +2968,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
>   		goto err_unlock;
>   	}
>   
> -
> -	if (p->user_cpus_ptr)
> -		not_empty = cpumask_and(new_mask, p->user_cpus_ptr, subset_mask);
> -	else
> -		not_empty = cpumask_and(new_mask, cpu_online_mask, subset_mask);
> -
> -	if (!not_empty) {
> +	if (!cpumask_and(new_mask, task_user_cpus(p), subset_mask)) {
>   		err = -EINVAL;
>   		goto err_unlock;
>   	}
> @@ -3044,16 +3037,11 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask, bool save
>    */
>   void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
>   {
> -	const struct cpumask *user_mask = p->user_cpus_ptr;
> -
> -	if (!user_mask)
> -		user_mask = cpu_online_mask;
> -
>   	/*
>   	 * Try to restore the old affinity mask with __sched_setaffinity().
>   	 * Cpuset masking will be done there too.
>   	 */
> -	__sched_setaffinity(p, user_mask, false);
> +	__sched_setaffinity(p, task_user_cpus(p), false);
>   }
>   
>   void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 15eefcd65faa..426e9b64b587 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1881,6 +1881,13 @@ static inline void dirty_sched_domain_sysctl(int cpu)
>   #endif
>   
>   extern int sched_update_scaling(void);
> +
> +static inline const struct cpumask *task_user_cpus(struct task_struct *p)
> +{
> +	if (!p->user_cpus_ptr)
> +		return cpus_possible_mask; /* &init_task.cpus_mask */
> +	return p->user_cpus_ptr;
> +}
>   #endif /* CONFIG_SMP */
>   
>   #include "stats.h"
Thanks for the good suggestions, will make the changes.

Cheers,
Longman