linux-kernel - Re: [PATCH 2/2] sched/deadline: Don't return invalid cpu in cpudl_maximum

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Date:   Fri, 9 Jun 2017 12:42:06 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH 2/2] sched/deadline: Don't return invalid cpu in
 cpudl_maximum_cpu()

On 09/06/17 11:43, Byungchul Park wrote:
> On Thu, Jun 08, 2017 at 03:02:43PM +0100, Juri Lelli wrote:
> > On 07/06/17 09:14, Byungchul Park wrote:
> > > On Wed, Jun 07, 2017 at 08:42:24AM +0900, Byungchul Park wrote:
> > > > On Tue, Jun 06, 2017 at 04:12:25PM +0100, Juri Lelli wrote:
> > > > > Hi,
> > > > > 
> > > > > On 02/06/17 16:31, Byungchul Park wrote:
> > 
> > [...]
> > 
> > > > > >  
> > > > > >  static inline int cpudl_maximum_cpu(struct cpudl *cp)
> > > > > >  {
> > > > > > -	return cp->elements[0].cpu;
> > > > > > +	int cpu = cp->elements[0].cpu;
> > > > > > +	return cp->elements[cpu].idx == IDX_INVALID ? -1 : cpu;
> > > > > 
> > > > > Mmm, don't we get a WARN from cpumask_check() if we return -1 here?
> > > > 
> > > > The function does not return -1 without my patch.
> > > > 
> > > > Right?
> > > 
> > 
> > That's actually my point: with the change you are proposing we will
> > start returning -1 and it looks to me that the WARN will start to fire.
> 
> Hi,
> 
> I see what you talk about. You are talking about WARN in cpumask_check().
> Sorry for missing your words.
> 
> > What about the below instead (properly splitted in 2 patches I guess,
> > and I'm not sure at all the macro thing is pretty at all) ?
> > 
> > --->8---
> >  kernel/sched/cpudeadline.c | 19 +++++++++++++------
> >  1 file changed, 13 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index fba235c7d026..32e3dcef2b81 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -108,11 +108,17 @@ static void cpudl_heapify(struct cpudl *cp, int idx)
> >  		cpudl_heapify_down(cp, idx);
> >  }
> >  
> > -static inline int cpudl_maximum(struct cpudl *cp)
> > -{
> > -	return cp->elements[0].cpu;
> > +#define cpudl_maximum(field)				\
> > +static inline int cpudl_maximum_##field			\
> > +(struct cpudl *cp)					\
> > +{							\
> > +	return cp->elements[0].field;			\
> >  }
> >  
> > +cpudl_maximum(cpu);
> > +cpudl_maximum(dl);
> > +cpudl_maximum(idx);
> > +
> >  /*
> >   * cpudl_find - find the best (later-dl) CPU in the system
> >   * @cp: the cpudl max-heap context
> > @@ -131,9 +137,10 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
> >  	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
> >  		best_cpu = cpumask_any(later_mask);
> >  		goto out;
> > -	} else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
> > -			dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
> > -		best_cpu = cpudl_maximum(cp);
> > +	} else if (cpudl_maximum_idx(cp) != IDX_INVALID &&
> > +		   cpumask_test_cpu(cpudl_maximum_cpu(cp), &p->cpus_allowed) &&
> > +		   dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
> > +		best_cpu = cpudl_maximum_cpu(cp);
> 
> This would also work and avoid unnecessary warning. I missed the check
> to avoid it. https://lkml.org/lkml/2017/3/23/175 was an original patch
> doing it.
> 
> By the way, frankly speaking, I don't like accessing the cpudl instant
> several times without protection. I rather prefer the following..
> 
> But whatever. I like both.
> 
> Thnaks,
> Byungchul
> 
> ----->8-----
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 9b314a9..1d369cf 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -137,11 +137,17 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
>  		best_cpu = cpumask_any(later_mask);
>  		goto out;
> -	} else if (cpumask_test_cpu(cpudl_maximum_cpu(cp), &p->cpus_allowed) &&
> -			dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
> -		best_cpu = cpudl_maximum_cpu(cp);
> -		if (later_mask)
> -			cpumask_set_cpu(best_cpu, later_mask);
> +	} else {
> +		int max_cpu = cpudl_maximum_cpu(cp);
> +		u64 max_dl = cpudl_maximum_dl(cp);
> +
> +		if (max_cpu != -1 &&
> +		    cpumask_test_cpu(max_cpu, &p->cpus_allowed) &&
> +		    dl_time_before(dl_se->deadline, max_dl)) {

Don't we access cp 3 times both ways?