[PATCH V3 07/10] cgroup: Constrain 'sched_load_balance' flag when DL tasks are present

Mathieu Poirier
2018-02-13T20:33:56Z

This patch prevents the 'sched_load_balance' flag from being set to 0
when DL tasks are present in a CPUset. Otherwise we end up with the
DL tasks using CPUs belonging to different root domains, something that
breaks the mathematical model behind DL bandwidth management.

For example on a 4 core system CPUset "set1" has been created and CPUs
0 and 1 assigned to it. A DL task has also been spun off. By default
the DL task can use all the CPUs in the default CPUset.

If we set the base CPUset's cpuset.sched_load_balance to 0, CPU 0 and 1
are added to a newly created root domain while CPU 2 and 3 endup in the
default root domain. But the DL task is still part of the base CPUset and
as such can use CPUs 0 to 3, spanning at the same time more than one root
domain.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
---
kernel/cgroup/cpuset.c | 104 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 104 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 45a5035ae601..4f5e8bac5337 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -458,6 +458,106 @@ static void free_trial_cpuset(struct cpuset *trial)
kfree(trial);
}

+static bool cpuset_has_dl_tasks(struct cpuset *cs)
+{
+ bool dl_tasks = false;
+ struct css_task_iter it;
+ struct task_struct *task;
+
+ /* Go through each task in @cs looking for a DL task */
+ css_task_iter_start(&cs->css, 0, &it);
+
+ while (!dl_tasks && (task = css_task_iter_next(&it))) {
+ if (dl_task(task))
+ dl_tasks = true;
+ }
+
+ css_task_iter_end(&it);
+
+ return dl_tasks;
+}
+
+/*
+ * Assumes RCU read lock and cpuset_mutex are held.
+ */
+static int
+validate_change_load_balance(struct cpuset *cur, struct cpuset *trial)
+{
+ bool populated = false, dl_tasks = false;
+ int ret = -EBUSY;
+ struct cgroup_subsys_state *pos_css;
+ struct cpuset *cs;
+
+ /* Bail out if nothing has changed. */
+ if (is_sched_load_balance(cur) ==
+ is_sched_load_balance(trial)) {
+ ret = 0;
+ goto out;
+ }
+
+ /*
+ * First deal with the generic case that applies when
+ * cpuset.sched_load_balance gets flipped on a cpuset,
+ * regardless of the value.
+ */
+ cpuset_for_each_descendant_pre(cs, pos_css, cur) {
+ if (cpuset_has_dl_tasks(cs))
+ dl_tasks = true;
+
+ /* Skip the top cpuset since it obviously exists */
+ if (cs == cur)
+ continue;
+
+ /* Children without CPUs are not important */
+ if (cpumask_empty(cs->cpus_allowed)) {
+ pos_css = css_rightmost_descendant(pos_css);
+ continue;
+ }
+
+ /* CPUs have been assigned to this cpuset. */
+ populated = true;
+
+ /*
+ * Go no further if both conditions are true so that we
+ * don't end up in a situation where a DL task is
+ * spanning more than one root domain or only assigned
+ * to a subset of the CPUs in a root domain.
+ */
+ if (populated && dl_tasks)
+ goto out;
+ }
+
+ /*
+ * Things get very complicated when dealing with children cpuset,
+ * resulting in hard to maintain code and low confidence that
+ * all cases are handled properly. As such prevent the
+ * cpuset.sched_load_balance from being modified on children cpuset
+ * where DL tasks have been assigned (or any of its children).
+ */
+ if (dl_tasks && parent_cs(cur))
+ goto out;
+
+ ret = 0;
+out:
+ return ret;
+}
+
+/*
+ * Assumes RCU read lock and cpuset_mutex are held.
+ */
+static int
+validate_dl_change(struct cpuset *cur, struct cpuset *trial)
+{
+ int ret = 0;
+
+ /* Check if the sched_load_balance flag has been changed */
+ ret = validate_change_load_balance(cur, trial);
+ if (ret)
+ return ret;
+
+ return ret;
+}
+
/*
* validate_change() - Used to validate that any proposed cpuset change
* follows the structural rules for cpusets.
@@ -492,6 +592,10 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
if (!is_cpuset_subset(c, trial))
goto out;

+ /* Make sure changes are compatible with deadline scheduling class */
+ if (validate_dl_change(cur, trial))
+ goto out;
+
/* Remaining checks don't apply to root cpuset */
ret = 0;
if (cur == &top_cpuset)
--
2.7.4