linux-kernel - [RFC PATCH 1/4] Minimally invasive deadline wrap-around bugfix for SCHED

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Date:	Tue, 19 Jul 2016 11:44:50 +0200
From:	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
To:	Luca Abeni <luca.abeni@...tn.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-dl@...is.sssup.it,
	Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
	Juri Lelli <juri.lelli@....com>
Subject: [RFC PATCH 1/4] Minimally invasive deadline wrap-around bugfix for SCHED_DEADLINE cpu heap

Current code in cpudeadline.c has a bug in re-heapifying when adding a
new element at the end of the heap, because a deadline value of 0 is
temporarily set in the new elem, then cpudl_change_key() is called
with the actual elem deadline as param. However, the function compares
the new deadline to set with the one previously in the elem, which is
0.  So, if current absolute deadlines grew so much to have negative
values as s64, the comparison in cpudl_change_key() makes the wrong
decision.  Instead, as from dl_time_before(), the kernel should handle
correctly abs deadlines wrap-arounds.

This patch fixes the problem with a minimally invasive change that
forces cpudl_change_key() to heapify up in this case.

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@....com>
Cc: Luca Abeni <luca.abeni@...tn.it>
Reviewed-by: Luca Abeni <luca.abeni@...tn.it>
Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
---
 kernel/sched/cpudeadline.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 5be5882..d418449 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -168,7 +168,7 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)

 	if (old_idx == IDX_INVALID) {
 		cp->size++;
-		cp->elements[cp->size - 1].dl = 0;
+		cp->elements[cp->size - 1].dl = dl;
 		cp->elements[cp->size - 1].cpu = cpu;
 		cp->elements[cpu].idx = cp->size - 1;
 		cpudl_change_key(cp, cp->size - 1, dl);
-- 
2.7.4