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, 43 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/procfs-diskstats b/Documentation/ABI/testing/procfs-diskstats
new file mode 100644
index 000000000..6a719cf20
--- /dev/null
+++ b/Documentation/ABI/testing/procfs-diskstats
@@ -0,0 +1,43 @@
+What:		/proc/diskstats
+Date:		February 2008
+Contact:	Jerome Marchand <jmarchan@redhat.com>
+Description:
+		The /proc/diskstats file displays the I/O statistics
+		of block devices. Each line contains the following 14
+		fields:
+
+		==  ===================================
+		 1  major number
+		 2  minor number
+		 3  device name
+		 4  reads completed successfully
+		 5  reads merged
+		 6  sectors read
+		 7  time spent reading (ms)
+		 8  writes completed
+		 9  writes merged
+		10  sectors written
+		11  time spent writing (ms)
+		12  I/Os currently in progress
+		13  time spent doing I/Os (ms)
+		14  weighted time spent doing I/Os (ms)
+		==  ===================================
+
+		Kernel 4.18+ appends four more fields for discard
+		tracking putting the total at 18:
+
+		==  ===================================
+		15  discards completed successfully
+		16  discards merged
+		17  sectors discarded
+		18  time spent discarding
+		==  ===================================
+
+		Kernel 5.5+ appends two more fields for flush requests:
+
+		==  =====================================
+		19  flush requests completed successfully
+		20  time spent flushing
+		==  =====================================
+
+		For more details refer to Documentation/admin-guide/iostats.rst