[PATCH v2 6/6] sched_ext: Make task_can_run_on_remote_rq() use common task_allowed_on_cpu()

Tejun Heo
2024-08-06T19:39:06Z

task_can_run_on_remote_rq() is similar to is_cpu_allowed() but there are
subtle differences. It currently open codes all the tests. This is
cumbersome to understand and error-prone in case the intersecting tests need
to be updated.

Factor out the common part - testing whether the task is allowed on the CPU
at all regardless of the CPU state - into task_allowed_on_cpu() and make
both is_cpu_allowed() and SCX's task_can_run_on_remote_rq() use it. As the
code is now linked between the two and each contains only the extra tests
that differ between them, it's less error-prone when the conditions need to
be updated. Also, improve the comment to explain why they are different.

v2: Replace accidental "extern inline" with "static inline" (Peter).

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: David Vernet <void@manifault.com>
---
kernel/sched/core.c | 4 ++--
kernel/sched/ext.c | 21 ++++++++++++++++-----
kernel/sched/sched.h | 18 ++++++++++++++++++
3 files changed, 36 insertions(+), 7 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2311,7 +2311,7 @@ static inline bool rq_has_pinned_tasks(s
static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
{
/* When not in the task's cpumask, no point in looking further. */
- if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+ if (!task_allowed_on_cpu(p, cpu))
return false;

/* migrate_disabled() must be allowed to finish. */
@@ -2320,7 +2320,7 @@ static inline bool is_cpu_allowed(struct

/* Non kernel threads are not allowed during either online or offline. */
if (!(p->flags & PF_KTHREAD))
- return cpu_active(cpu) && task_cpu_possible(cpu, p);
+ return cpu_active(cpu);

/* KTHREAD_IS_PER_CPU is always allowed. */
if (kthread_is_per_cpu(p))
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2224,19 +2224,30 @@ static void consume_local_task(struct rq

#ifdef CONFIG_SMP
/*
- * Similar to kernel/sched/core.c::is_cpu_allowed() but we're testing whether @p
- * can be pulled to @rq.
+ * Similar to kernel/sched/core.c::is_cpu_allowed(). However, there are two
+ * differences:
+ *
+ * - is_cpu_allowed() asks "Can this task run on this CPU?" while
+ * task_can_run_on_remote_rq() asks "Can the BPF scheduler migrate the task to
+ * this CPU?".
+ *
+ * While migration is disabled, is_cpu_allowed() has to say "yes" as the task
+ * must be allowed to finish on the CPU that it's currently on regardless of
+ * the CPU state. However, task_can_run_on_remote_rq() must say "no" as the
+ * BPF scheduler shouldn't attempt to migrate a task which has migration
+ * disabled.
+ *
+ * - The BPF scheduler is bypassed while the rq is offline and we can always say
+ * no to the BPF scheduler initiated migrations while offline.
*/
static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq)
{
int cpu = cpu_of(rq);

- if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+ if (!task_allowed_on_cpu(p, cpu))
return false;
if (unlikely(is_migration_disabled(p)))
return false;
- if (!(p->flags & PF_KTHREAD) && unlikely(!task_cpu_possible(cpu, p)))
- return false;
if (!scx_rq_online(rq))
return false;
return true;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2530,6 +2530,19 @@ extern void sched_balance_trigger(struct
extern int __set_cpus_allowed_ptr(struct task_struct *p, struct affinity_context *ctx);
extern void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx);

+static inline bool task_allowed_on_cpu(struct task_struct *p, int cpu)
+{
+ /* When not in the task's cpumask, no point in looking further. */
+ if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+ return false;
+
+ /* Can @cpu run a user thread? */
+ if (!(p->flags & PF_KTHREAD) && !task_cpu_possible(cpu, p))
+ return false;
+
+ return true;
+}
+
static inline cpumask_t *alloc_user_cpus_ptr(int node)
{
/*
@@ -2563,6 +2576,11 @@ extern int push_cpu_stop(void *arg);

#else /* !CONFIG_SMP: */

+static inline bool task_allowed_on_cpu(struct task_struct *p, int cpu)
+{
+ return true;
+}
+
static inline int __set_cpus_allowed_ptr(struct task_struct *p,
struct affinity_context *ctx)
{