Re: [PATCH v6 12/21] sched: Allow task CPU affinity to be restricted on asymmetric systems

[Will Deacon <will@kernel.org>]

On Fri, May 21, 2021 at 06:11:32PM +0100, Qais Yousef wrote:
> On 05/18/21 10:47, Will Deacon wrote:
> > +static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
> > +					 const struct cpumask *new_mask,
> > +					 u32 flags,
> > +					 struct rq *rq,
> > +					 struct rq_flags *rf)
> > +	__releases(rq->lock)
> > +	__releases(p->pi_lock)
> >  {
> >  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
> >  	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
> >  	unsigned int dest_cpu;
> > -	struct rq_flags rf;
> > -	struct rq *rq;
> >  	int ret = 0;
> >  
> > -	rq = task_rq_lock(p, &rf);
> >  	update_rq_clock(rq);
> >  
> >  	if (p->flags & PF_KTHREAD || is_migration_disabled(p)) {
> > @@ -2430,20 +2425,158 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  
> >  	__do_set_cpus_allowed(p, new_mask, flags);
> >  
> > -	return affine_move_task(rq, p, &rf, dest_cpu, flags);
> > +	if (flags & SCA_USER)
> > +		release_user_cpus_ptr(p);
> 
> Why do we need to release the pointer here?
> 
> Doesn't this mean if a 32bit task requests to change its affinity, then we'll
> lose this info and a subsequent execve() to a 64bit application means we won't
> be able to restore the original mask?
> 
> ie:
> 
> 	p0-64bit
> 	  execve(32bit_app)
> 	    // p1-32bit created
> 	    p1-32bit.change_affinity()
> 	      relase_user_cpus_ptr()
> 	    execve(64bit_app)           // lost info about p0 affinity?
> 
> Hmm I think this helped me to get the answer. p1 changed its affinity, then
> there's nothing to be inherited by a new execve(), so yes we no longer need
> this info.

Yup, you got it.

> > +static int restrict_cpus_allowed_ptr(struct task_struct *p,
> > +				     struct cpumask *new_mask,
> > +				     const struct cpumask *subset_mask)
> > +{
> > +	struct rq_flags rf;
> > +	struct rq *rq;
> > +	int err;
> > +	struct cpumask *user_mask = NULL;
> > +
> > +	if (!p->user_cpus_ptr)
> > +		user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
> > +
> > +	rq = task_rq_lock(p, &rf);
> > +
> > +	/*
> > +	 * 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;
> > +	}
> > +
> > +	/*
> > +	 * Forcefully restricting the affinity of a deadline task is
> > +	 * likely to cause problems, so fail and noisily override the
> > +	 * mask entirely.
> > +	 */
> > +	if (task_has_dl_policy(p) && dl_bandwidth_enabled()) {
> > +		err = -EPERM;
> > +		goto err_unlock;
> 
> free(user_mark) first?
> 
> > +	}
> > +
> > +	if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) {
> > +		err = -EINVAL;
> > +		goto err_unlock;
> 
> ditto

We free the mask when the task exits so we don't actually need to clean up
here. I left it like this on the assumption that failing here means that
it's very likely that either the task will exit or retry very soon.

However I agree that it would be clearer to free the thing anyway, so I'll
rejig the code to do that.

Will