[PATCH v7 09/22] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()

Will Deacon
2021-05-25T15:21:50Z

Asymmetric systems may not offer the same level of userspace ISA support
across all CPUs, meaning that some applications cannot be executed by
some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
not feature support for 32-bit applications on both clusters.

Modify guarantee_online_cpus() to take task_cpu_possible_mask() into
account when trying to find a suitable set of online CPUs for a given
task. This will avoid passing an invalid mask to set_cpus_allowed_ptr()
during ->attach() and will subsequently allow the cpuset hierarchy to be
taken into account when forcefully overriding the affinity mask for a
task which requires migration to a compatible CPU.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
include/linux/cpuset.h | 2 +-
kernel/cgroup/cpuset.c | 41 ++++++++++++++++++++++++++---------------
2 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index ed6ec677dd6b..414a8e694413 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -185,7 +185,7 @@ static inline void cpuset_read_unlock(void) { }
static inline void cpuset_cpus_allowed(struct task_struct *p,
struct cpumask *mask)
{
- cpumask_copy(mask, cpu_possible_mask);
+ cpumask_copy(mask, task_cpu_possible_mask(p));
}

static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8c799260a4a2..e0649c970f48 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -372,18 +372,29 @@ static inline bool is_in_v2_mode(void)
}

/*
- * Return in pmask the portion of a cpusets's cpus_allowed that
- * are online. If none are online, walk up the cpuset hierarchy
- * until we find one that does have some online cpus.
+ * Return in pmask the portion of a task's cpusets's cpus_allowed that
+ * are online and are capable of running the task. If none are found,
+ * walk up the cpuset hierarchy until we find one that does have some
+ * appropriate cpus.
*
* One way or another, we guarantee to return some non-empty subset
* of cpu_online_mask.
*
* Call with callback_lock or cpuset_mutex held.
*/
-static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
+static void guarantee_online_cpus(struct task_struct *tsk,
+ struct cpumask *pmask)
{
- while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask)) {
+ const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
+ struct cpuset *cs;
+
+ if (WARN_ON(!cpumask_and(pmask, possible_mask, cpu_online_mask)))
+ cpumask_copy(pmask, cpu_online_mask);
+
+ rcu_read_lock();
+ cs = task_cs(tsk);
+
+ while (!cpumask_intersects(cs->effective_cpus, pmask)) {
cs = parent_cs(cs);
if (unlikely(!cs)) {
/*
@@ -393,11 +404,13 @@ static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
* cpuset's effective_cpus is on its way to be
* identical to cpu_online_mask.
*/
- cpumask_copy(pmask, cpu_online_mask);
- return;
+ goto out_unlock;
}
}
- cpumask_and(pmask, cs->effective_cpus, cpu_online_mask);
+ cpumask_and(pmask, pmask, cs->effective_cpus);
+
+out_unlock:
+ rcu_read_unlock();
}

/*
@@ -2199,15 +2212,13 @@ static void cpuset_attach(struct cgroup_taskset *tset)

percpu_down_write(&cpuset_rwsem);

- /* prepare for attach */
- if (cs == &top_cpuset)
- cpumask_copy(cpus_attach, cpu_possible_mask);
- else
- guarantee_online_cpus(cs, cpus_attach);
-
guarantee_online_mems(cs, &cpuset_attach_nodemask_to);

cgroup_taskset_for_each(task, css, tset) {
+ if (cs != &top_cpuset)
+ guarantee_online_cpus(task, cpus_attach);
+ else
+ cpumask_copy(cpus_attach, task_cpu_possible_mask(task));
/*
* can_attach beforehand should guarantee that this doesn't
* fail. TODO: have a better way to handle failure here
@@ -3303,7 +3314,7 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)

spin_lock_irqsave(&callback_lock, flags);
rcu_read_lock();
- guarantee_online_cpus(task_cs(tsk), pmask);
+ guarantee_online_cpus(tsk, pmask);
rcu_read_unlock();
spin_unlock_irqrestore(&callback_lock, flags);
}
--
2.31.1.818.g46aad6cb9e-goog