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, 73 insertions, 0 deletions

diff --git a/include/uapi/asm-generic/stat.h b/include/uapi/asm-generic/stat.h
new file mode 100644
index 000000000..0d962ecd1
--- /dev/null
+++ b/include/uapi/asm-generic/stat.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef __ASM_GENERIC_STAT_H
+#define __ASM_GENERIC_STAT_H
+
+/*
+ * Everybody gets this wrong and has to stick with it for all
+ * eternity. Hopefully, this version gets used by new architectures
+ * so they don't fall into the same traps.
+ *
+ * stat64 is copied from powerpc64, with explicit padding added.
+ * stat is the same structure layout on 64-bit, without the 'long long'
+ * types.
+ *
+ * By convention, 64 bit architectures use the stat interface, while
+ * 32 bit architectures use the stat64 interface. Note that we don't
+ * provide an __old_kernel_stat here, which new architecture should
+ * not have to start with.
+ */
+
+#include <asm/bitsperlong.h>
+
+#define STAT_HAVE_NSEC 1
+
+struct stat {
+	unsigned long	st_dev;		/* Device.  */
+	unsigned long	st_ino;		/* File serial number.  */
+	unsigned int	st_mode;	/* File mode.  */
+	unsigned int	st_nlink;	/* Link count.  */
+	unsigned int	st_uid;		/* User ID of the file's owner.  */
+	unsigned int	st_gid;		/* Group ID of the file's group. */
+	unsigned long	st_rdev;	/* Device number, if device.  */
+	unsigned long	__pad1;
+	long		st_size;	/* Size of file, in bytes.  */
+	int		st_blksize;	/* Optimal block size for I/O.  */
+	int		__pad2;
+	long		st_blocks;	/* Number 512-byte blocks allocated. */
+	long		st_atime;	/* Time of last access.  */
+	unsigned long	st_atime_nsec;
+	long		st_mtime;	/* Time of last modification.  */
+	unsigned long	st_mtime_nsec;
+	long		st_ctime;	/* Time of last status change.  */
+	unsigned long	st_ctime_nsec;
+	unsigned int	__unused4;
+	unsigned int	__unused5;
+};
+
+/* This matches struct stat64 in glibc2.1. Only used for 32 bit. */
+#if __BITS_PER_LONG != 64 || defined(__ARCH_WANT_STAT64)
+struct stat64 {
+	unsigned long long st_dev;	/* Device.  */
+	unsigned long long st_ino;	/* File serial number.  */
+	unsigned int	st_mode;	/* File mode.  */
+	unsigned int	st_nlink;	/* Link count.  */
+	unsigned int	st_uid;		/* User ID of the file's owner.  */
+	unsigned int	st_gid;		/* Group ID of the file's group. */
+	unsigned long long st_rdev;	/* Device number, if device.  */
+	unsigned long long __pad1;
+	long long	st_size;	/* Size of file, in bytes.  */
+	int		st_blksize;	/* Optimal block size for I/O.  */
+	int		__pad2;
+	long long	st_blocks;	/* Number 512-byte blocks allocated. */
+	int		st_atime;	/* Time of last access.  */
+	unsigned int	st_atime_nsec;
+	int		st_mtime;	/* Time of last modification.  */
+	unsigned int	st_mtime_nsec;
+	int		st_ctime;	/* Time of last status change.  */
+	unsigned int	st_ctime_nsec;
+	unsigned int	__unused4;
+	unsigned int	__unused5;
+};
+#endif
+
+#endif /* __ASM_GENERIC_STAT_H */