sched_ext: Improve error reporting during loading

When the BPF scheduler fails, ops.exit() allows rich error reporting through
scx_exit_info. Use scx.exit() path consistently for all failures which can
be caused by the BPF scheduler:

- scx_ops_error() is called after ops.init() and ops.cgroup_init() failure
  to record error information.

- ops.init_task() failure now uses scx_ops_error() instead of pr_err().

- The err_disable path updated to automatically trigger scx_ops_error() to
  cover cases that the error message hasn't already been generated and
  always return 0 indicating init success so that the error is reported
  through ops.exit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Vernet <void@manifault.com>
Cc: Daniel Hodges <hodges.daniel.scott@gmail.com>
Cc: Changwoo Min <multics69@gmail.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 3cd7c50..76f04f3 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -625,6 +625,10 @@ struct sched_ext_ops {
        /**
         * exit - Clean up after the BPF scheduler
         * @info: Exit info
+        *
+        * ops.exit() is also called on ops.init() failure, which is a bit
+        * unusual. This is to allow rich reporting through @info on how
+        * ops.init() failed.
         */
        void (*exit)(struct scx_exit_info *info);
 
@@ -4117,6 +4121,7 @@ static int scx_cgroup_init(void)
                                      css->cgroup, &args);
                if (ret) {
                        css_put(css);
+                       scx_ops_error("ops.cgroup_init() failed (%d)", ret);
                        return ret;
                }
                tg->scx_flags |= SCX_TG_INITED;
@@ -5041,6 +5046,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
                if (ret) {
                        ret = ops_sanitize_err("init", ret);
                        cpus_read_unlock();
+                       scx_ops_error("ops.init() failed (%d)", ret);
                        goto err_disable;
                }
        }
@@ -5150,8 +5156,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
                        spin_lock_irq(&scx_tasks_lock);
                        scx_task_iter_exit(&sti);
                        spin_unlock_irq(&scx_tasks_lock);
-                       pr_err("sched_ext: ops.init_task() failed (%d) for %s[%d] while loading\n",
-                              ret, p->comm, p->pid);
+                       scx_ops_error("ops.init_task() failed (%d) for %s[%d]",
+                                     ret, p->comm, p->pid);
                        goto err_disable_unlock_all;
                }
 
@@ -5199,14 +5205,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 
        scx_ops_bypass(false);
 
-       /*
-        * Returning an error code here would lose the recorded error
-        * information. Exit indicating success so that the error is notified
-        * through ops.exit() with all the details.
-        */
        if (!scx_ops_tryset_enable_state(SCX_OPS_ENABLED, SCX_OPS_ENABLING)) {
                WARN_ON_ONCE(atomic_read(&scx_exit_kind) == SCX_EXIT_NONE);
-               ret = 0;
                goto err_disable;
        }
 
@@ -5241,10 +5241,18 @@ err_disable_unlock_all:
        scx_ops_bypass(false);
 err_disable:
        mutex_unlock(&scx_ops_enable_mutex);
-       /* must be fully disabled before returning */
-       scx_ops_disable(SCX_EXIT_ERROR);
+       /*
+        * Returning an error code here would not pass all the error information
+        * to userspace. Record errno using scx_ops_error() for cases
+        * scx_ops_error() wasn't already invoked and exit indicating success so
+        * that the error is notified through ops.exit() with all the details.
+        *
+        * Flush scx_ops_disable_work to ensure that error is reported before
+        * init completion.
+        */
+       scx_ops_error("scx_ops_enable() failed (%d)", ret);
        kthread_flush_work(&scx_ops_disable_work);
-       return ret;
+       return 0;
 }