sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/include/drm/drm_utils.h b/include/drm/drm_utils.h
new file mode 100644
index 000000000..7e077484c
--- /dev/null
+++ b/include/drm/drm_utils.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Function prototypes for misc. drm utility functions.
+ * Specifically this file is for function prototypes for functions which
+ * may also be used outside of drm code (e.g. in fbdev drivers).
+ *
+ * Copyright (C) 2017 Hans de Goede <hdegoede@redhat.com>
+ */
+
+#ifndef __DRM_UTILS_H__
+#define __DRM_UTILS_H__
+
+#include <linux/types.h>
+
+struct drm_edid;
+
+int drm_get_panel_orientation_quirk(int width, int height);
+
+struct drm_panel_backlight_quirk {
+	u16 min_brightness;
+	u32 brightness_mask;
+	bool force_pwm;
+};
+
+const struct drm_panel_backlight_quirk *
+drm_get_panel_backlight_quirk(const struct drm_edid *edid);
+
+signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec);
+
+#endif