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, 62 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-mapping b/Documentation/ABI/testing/sysfs-devices-mapping
new file mode 100644
index 000000000..2eee1446a
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-devices-mapping
@@ -0,0 +1,62 @@
+What:           /sys/devices/uncore_iio_x/dieX
+Date:           February 2020
+Contact:        Alexander Antonov <alexander.antonov@linux.intel.com>
+Description:
+                Each IIO stack (PCIe root port) has its own IIO PMON block, so
+                each dieX file (where X is die number) holds "Segment:Root Bus"
+                for PCIe root port, which can be monitored by that IIO PMON
+                block.
+                For example, on 4-die Xeon platform with up to 6 IIO stacks per
+                die and, therefore, 6 IIO PMON blocks per die, the mapping of
+                IIO PMON block 0 exposes as the following::
+
+		    $ ls /sys/devices/uncore_iio_0/die*
+		    -r--r--r-- /sys/devices/uncore_iio_0/die0
+		    -r--r--r-- /sys/devices/uncore_iio_0/die1
+		    -r--r--r-- /sys/devices/uncore_iio_0/die2
+		    -r--r--r-- /sys/devices/uncore_iio_0/die3
+
+		    $ tail /sys/devices/uncore_iio_0/die*
+		    ==> /sys/devices/uncore_iio_0/die0 <==
+		    0000:00
+		    ==> /sys/devices/uncore_iio_0/die1 <==
+		    0000:40
+		    ==> /sys/devices/uncore_iio_0/die2 <==
+		    0000:80
+		    ==> /sys/devices/uncore_iio_0/die3 <==
+		    0000:c0
+
+                Which means::
+
+		    IIO PMU 0 on die 0 belongs to PCI RP on bus 0x00, domain 0x0000
+		    IIO PMU 0 on die 1 belongs to PCI RP on bus 0x40, domain 0x0000
+		    IIO PMU 0 on die 2 belongs to PCI RP on bus 0x80, domain 0x0000
+		    IIO PMU 0 on die 3 belongs to PCI RP on bus 0xc0, domain 0x0000
+
+What:           /sys/devices/uncore_upi_x/dieX
+Date:           March 2022
+Contact:        Alexander Antonov <alexander.antonov@linux.intel.com>
+Description:
+                Each /sys/devices/uncore_upi_X/dieY file holds "upi_Z,die_W"
+                value that means UPI link number X on die Y is connected to UPI
+                link Z on die W and this link between sockets can be monitored
+                by UPI PMON block.
+                For example, 4-die Sapphire Rapids platform has the following
+                UPI 0 topology::
+
+		    # tail /sys/devices/uncore_upi_0/die*
+		    ==> /sys/devices/uncore_upi_0/die0 <==
+		    upi_1,die_1
+		    ==> /sys/devices/uncore_upi_0/die1 <==
+		    upi_0,die_3
+		    ==> /sys/devices/uncore_upi_0/die2 <==
+		    upi_1,die_3
+		    ==> /sys/devices/uncore_upi_0/die3 <==
+		    upi_0,die_1
+
+                Which means::
+
+		    UPI link 0 on die 0 is connected to UPI link 1 on die 1
+		    UPI link 0 on die 1 is connected to UPI link 0 on die 3
+		    UPI link 0 on die 2 is connected to UPI link 1 on die 3
+		    UPI link 0 on die 3 is connected to UPI link 0 on die 1
\ No newline at end of file