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

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Date:   Thu, 8 Jun 2017 15:02:43 +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 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.

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);
 		if (later_mask)
 			cpumask_set_cpu(best_cpu, later_mask);
 	}