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Date: Mon, 21 Aug 2017 16:21:28 +0100
From: Brendan Jackman <brendan.jackman@....com>
To: linux-kernel@...r.kernel.org
Cc: Joel Fernandes <joelaf@...gle.com>,
Andres Oportus <andresoportus@...gle.com>,
Ingo Molnar <mingo@...hat.com>,
Morten Rasmussen <morten.rasmussen@....com>,
Peter Zijlstra <peterz@...radead.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 2/2] sched/fair: Fix use of NULL with find_idlest_group
The current use of returning NULL from find_idlest_group is broken in
two cases:
a1) The local group is not allowed.
In this case, we currently do not change this_runnable_load or
this_avg_load from its initial value of 0, which means we return
NULL regardless of the load of the other, allowed groups. This
results in pointlessly continuing the find_idlest_group search
within the local group and then returning prev_cpu from
select_task_rq_fair.
a2) No CPUs in the sched_domain are allowed.
In this case we also return NULL and again pointlessly continue
the search.
b) smp_processor_id() is the "idlest" and != prev_cpu.
find_idlest_group also returns NULL when the local group is
allowed and is the idlest. The caller then continues the
find_idlest_group search at a lower level of the current CPU's
sched_domain hierarchy. However new_cpu is not updated. This means
the search is pointless and we return prev_cpu from
select_task_rq_fair.
This is fixed by:
1. Returning NULL from find_idlest_group only when _no_ groups were
allowed in the current sched_domain. In this case, we now break
from the while(sd) loop and immediately return prev_cpu. This
fixes case a2).
2. Initializing this_runnable_load and this_avg_load to ULONG_MAX
instead of 0. This means in case a1) we now return the idlest
non-local group.
3. Explicitly updating new_cpu when find_idlest_group returns the
local group, fixing case b).
This patch also re-words the check for whether the group in
consideration is local, under the assumption that the first group in
the sched domain is always the local one.
Signed-off-by: Brendan Jackman <brendan.jackman@....com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
---
kernel/sched/fair.c | 34 +++++++++++++++++++---------------
1 file changed, 19 insertions(+), 15 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 64618d768546..7cb5ed719cf9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5382,26 +5382,29 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
* domain.
*/
static struct sched_group *
-find_idlest_group(struct sched_domain *sd, struct task_struct *p,
- int this_cpu, int sd_flag)
+find_idlest_group(struct sched_domain *sd, struct task_struct *p, int sd_flag)
{
struct sched_group *idlest = NULL, *group = sd->groups;
+ struct sched_group *local_group = sd->groups;
struct sched_group *most_spare_sg = NULL;
- unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = 0;
- unsigned long min_avg_load = ULONG_MAX, this_avg_load = 0;
+ unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = ULONG_MAX;
+ unsigned long min_avg_load = ULONG_MAX, this_avg_load = ULONG_MAX;
unsigned long most_spare = 0, this_spare = 0;
int load_idx = sd->forkexec_idx;
int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;
unsigned long imbalance = scale_load_down(NICE_0_LOAD) *
(sd->imbalance_pct-100) / 100;
+ if (!cpumask_intersects(sched_domain_span(sd), &p->cpus_allowed))
+ return NULL;
+
if (sd_flag & SD_BALANCE_WAKE)
load_idx = sd->wake_idx;
do {
unsigned long load, avg_load, runnable_load;
unsigned long spare_cap, max_spare_cap;
- int local_group;
+ bool group_is_local = group == local_group;
int i;
/* Skip over this group if it has no CPUs allowed */
@@ -5409,9 +5412,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
&p->cpus_allowed))
continue;
- local_group = cpumask_test_cpu(this_cpu,
- sched_group_span(group));
-
/*
* Tally up the load of all CPUs in the group and find
* the group containing the CPU with most spare capacity.
@@ -5422,7 +5422,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
for_each_cpu(i, sched_group_span(group)) {
/* Bias balancing toward cpus of our domain */
- if (local_group)
+ if (group_is_local)
load = source_load(i, load_idx);
else
load = target_load(i, load_idx);
@@ -5443,7 +5443,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
group->sgc->capacity;
- if (local_group) {
+ if (group_is_local) {
this_runnable_load = runnable_load;
this_avg_load = avg_load;
this_spare = max_spare_cap;
@@ -5489,21 +5489,21 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
if (this_spare > task_util(p) / 2 &&
imbalance_scale*this_spare > 100*most_spare)
- return NULL;
+ return local_group;
if (most_spare > task_util(p) / 2)
return most_spare_sg;
skip_spare:
if (!idlest)
- return NULL;
+ return local_group;
if (min_runnable_load > (this_runnable_load + imbalance))
- return NULL;
+ return local_group;
if ((this_runnable_load < (min_runnable_load + imbalance)) &&
(100*this_avg_load < imbalance_scale*min_avg_load))
- return NULL;
+ return local_group;
return idlest;
}
@@ -5927,8 +5927,12 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
continue;
}
- group = find_idlest_group(sd, p, cpu, sd_flag);
+ group = find_idlest_group(sd, p, sd_flag);
if (!group) {
+ break;
+ } else if (group == sd->groups) {
+ new_cpu = cpu;
+ /* Now try balancing at a lower domain level of cpu */
sd = sd->child;
continue;
}
--
2.14.1
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