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sched_ext: Decouple locks in scx_ops_disable_workfn()

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The disable path uses three big locks - scx_fork_rwsem, scx_cgroup_rwsem and
cpus_read_lock. Currently, the locks are grabbed together which is prone to
locking order problems. With the preceding scx_cgroup_enabled change, we can
decouple them:

- As cgroup disabling no longer requires modifying a static_key which
  requires cpus_read_lock(), no need to grab cpus_read_lock() before
  grabbing scx_cgroup_rwsem.

- cgroup can now be independently disabled before tasks are moved back to
  the fair class.

Relocate scx_cgroup_exit() invocation before scx_fork_rwsem is grabbed, drop
now unnecessary cpus_read_lock() and move static_key operations out of
scx_fork_rwsem. This decouples all three locks in the disable path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Link: http://lkml.kernel.org/r/8cd0ec0c4c7c1bc0119e61fbef0bee9d5e24022d.camel@linux.ibm.com
This commit is contained in:
Tejun Heo 2024-09-27 10:02:40 -10:00
parent 568894edbe
commit 160216568c
1 changed files with 17 additions and 20 deletions
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37
kernel/sched/ext.c
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@ -4456,21 +4456,23 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
WRITE_ONCE(scx_switching_all, false);
/*
* Avoid racing against fork and cgroup changes. See scx_ops_enable()
* for explanation on the locking order.
* Shut down cgroup support before tasks so that the cgroup attach path
* doesn't race against scx_ops_exit_task().
*/
scx_cgroup_lock();
scx_cgroup_exit();
scx_cgroup_unlock();
/*
* The BPF scheduler is going away. All tasks including %TASK_DEAD ones
* must be switched out and exited synchronously.
*/
percpu_down_write(&scx_fork_rwsem);
cpus_read_lock();
scx_cgroup_lock();
scx_ops_init_task_enabled = false;
spin_lock_irq(&scx_tasks_lock);
scx_task_iter_init(&sti);
/*
* The BPF scheduler is going away. All tasks including %TASK_DEAD ones
* must be switched out and exited synchronously.
*/
while ((p = scx_task_iter_next_locked(&sti))) {
const struct sched_class *old_class = p->sched_class;
struct sched_enq_and_set_ctx ctx;
@ -4488,23 +4490,18 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
}
scx_task_iter_exit(&sti);
spin_unlock_irq(&scx_tasks_lock);
percpu_up_write(&scx_fork_rwsem);
/* no task is on scx, turn off all the switches and flush in-progress calls */
static_branch_disable_cpuslocked(&__scx_ops_enabled);
static_branch_disable(&__scx_ops_enabled);
for (i = SCX_OPI_BEGIN; i < SCX_OPI_END; i++)
static_branch_disable_cpuslocked(&scx_has_op[i]);
static_branch_disable_cpuslocked(&scx_ops_enq_last);
static_branch_disable_cpuslocked(&scx_ops_enq_exiting);
static_branch_disable_cpuslocked(&scx_ops_cpu_preempt);
static_branch_disable_cpuslocked(&scx_builtin_idle_enabled);
static_branch_disable(&scx_has_op[i]);
static_branch_disable(&scx_ops_enq_last);
static_branch_disable(&scx_ops_enq_exiting);
static_branch_disable(&scx_ops_cpu_preempt);
static_branch_disable(&scx_builtin_idle_enabled);
synchronize_rcu();
scx_cgroup_exit();
scx_cgroup_unlock();
cpus_read_unlock();
percpu_up_write(&scx_fork_rwsem);
if (ei->kind >= SCX_EXIT_ERROR) {
pr_err("sched_ext: BPF scheduler \"%s\" disabled (%s)\n",
scx_ops.name, ei->reason);