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, 133 insertions, 0 deletions

diff --git a/arch/alpha/lib/memset.S b/arch/alpha/lib/memset.S
new file mode 100644
index 000000000..9075d6918
--- /dev/null
+++ b/arch/alpha/lib/memset.S
@@ -0,0 +1,133 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * linux/arch/alpha/lib/memset.S
+ *
+ * This is an efficient (and small) implementation of the C library "memset()"
+ * function for the alpha.
+ *
+ *	(C) Copyright 1996 Linus Torvalds
+ *
+ * This routine is "moral-ware": you are free to use it any way you wish, and
+ * the only obligation I put on you is a moral one: if you make any improvements
+ * to the routine, please send me your improvements for me to use similarly.
+ *
+ * The scheduling comments are according to the EV5 documentation (and done by 
+ * hand, so they might well be incorrect, please do tell me about it..)
+ */
+#include <linux/export.h>
+	.set noat
+	.set noreorder
+.text
+	.globl memset
+	.globl __memset
+	.globl ___memset
+	.globl __memset16
+	.globl __constant_c_memset
+
+	.ent ___memset
+.align 5
+___memset:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	and $17,255,$1		/* E1 */
+	insbl $17,1,$17		/* .. E0 */
+	bis $17,$1,$17		/* E0 (p-c latency, next cycle) */
+	sll $17,16,$1		/* E1 (p-c latency, next cycle) */
+
+	bis $17,$1,$17		/* E0 (p-c latency, next cycle) */
+	sll $17,32,$1		/* E1 (p-c latency, next cycle) */
+	bis $17,$1,$17		/* E0 (p-c latency, next cycle) */
+	ldq_u $31,0($30)	/* .. E1 */
+
+.align 5
+__constant_c_memset:
+	addq $18,$16,$6		/* E0 */
+	bis $16,$16,$0		/* .. E1 */
+	xor $16,$6,$1		/* E0 */
+	ble $18,end		/* .. E1 */
+
+	bic $1,7,$1		/* E0 */
+	beq $1,within_one_quad	/* .. E1 (note EV5 zero-latency forwarding) */
+	and $16,7,$3		/* E0 */
+	beq $3,aligned		/* .. E1 (note EV5 zero-latency forwarding) */
+
+	ldq_u $4,0($16)		/* E0 */
+	bis $16,$16,$5		/* .. E1 */
+	insql $17,$16,$2	/* E0 */
+	subq $3,8,$3		/* .. E1 */
+
+	addq $18,$3,$18		/* E0	$18 is new count ($3 is negative) */
+	mskql $4,$16,$4		/* .. E1 (and possible load stall) */
+	subq $16,$3,$16		/* E0 	$16 is new aligned destination */
+	bis $2,$4,$1		/* .. E1 */
+
+	bis $31,$31,$31		/* E0 */
+	ldq_u $31,0($30)	/* .. E1 */
+	stq_u $1,0($5)		/* E0 */
+	bis $31,$31,$31		/* .. E1 */
+
+.align 4
+aligned:
+	sra $18,3,$3		/* E0 */
+	and $18,7,$18		/* .. E1 */
+	bis $16,$16,$5		/* E0 */
+	beq $3,no_quad		/* .. E1 */
+
+.align 3
+loop:
+	stq $17,0($5)		/* E0 */
+	subq $3,1,$3		/* .. E1 */
+	addq $5,8,$5		/* E0 */
+	bne $3,loop		/* .. E1 */
+
+no_quad:
+	bis $31,$31,$31		/* E0 */
+	beq $18,end		/* .. E1 */
+	ldq $7,0($5)		/* E0 */
+	mskqh $7,$6,$2		/* .. E1 (and load stall) */
+
+	insqh $17,$6,$4		/* E0 */
+	bis $2,$4,$1		/* .. E1 */
+	stq $1,0($5)		/* E0 */
+	ret $31,($26),1		/* .. E1 */
+
+.align 3
+within_one_quad:
+	ldq_u $1,0($16)		/* E0 */
+	insql $17,$16,$2	/* E1 */
+	mskql $1,$16,$4		/* E0 (after load stall) */
+	bis $2,$4,$2		/* E0 */
+
+	mskql $2,$6,$4		/* E0 */
+	mskqh $1,$6,$2		/* .. E1 */
+	bis $2,$4,$1		/* E0 */
+	stq_u $1,0($16)		/* E0 */
+
+end:
+	ret $31,($26),1		/* E1 */
+	.end ___memset
+EXPORT_SYMBOL(___memset)
+EXPORT_SYMBOL(__constant_c_memset)
+
+	.align 5
+	.ent __memset16
+__memset16:
+	.prologue 0
+
+	inswl $17,0,$1		/* E0 */
+	inswl $17,2,$2		/* E0 */
+	inswl $17,4,$3		/* E0 */
+	or $1,$2,$1		/* .. E1 */
+	inswl $17,6,$4		/* E0 */
+	or $1,$3,$1		/* .. E1 */
+	or $1,$4,$17		/* E0 */
+	br __constant_c_memset	/* .. E1 */
+
+	.end __memset16
+EXPORT_SYMBOL(__memset16)
+
+memset = ___memset
+__memset = ___memset
+	EXPORT_SYMBOL(memset)
+	EXPORT_SYMBOL(__memset)