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, 166 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-tty b/Documentation/ABI/testing/sysfs-tty
new file mode 100644
index 000000000..895c47f05
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-tty
@@ -0,0 +1,166 @@
+What:		/sys/class/tty/console/active
+Date:		Nov 2010
+Contact:	Kay Sievers <kay.sievers@vrfy.org>
+Description:
+		 Shows the list of currently configured
+		 console devices, like 'tty1 ttyS0'.
+		 The last entry in the file is the active
+		 device connected to /dev/console.
+		 The file supports poll() to detect virtual
+		 console switches.
+
+What:		/sys/class/tty/tty<x>/active
+Date:		Nov 2010
+Contact:	Kay Sievers <kay.sievers@vrfy.org>
+Description:
+		 Shows the currently active virtual console
+		 device, like 'tty1'.
+		 The file supports poll() to detect virtual
+		 console switches.
+
+What:		/sys/class/tty/ttyS<x>/uartclk
+Date:		Sep 2012
+Contact:	Tomas Hlavacek <tmshlvck@gmail.com>
+Description:
+		 Shows the current uartclk value associated with the
+		 UART port in serial_core, that is bound to TTY like ttyS0.
+		 uartclk = 16 * baud_base
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/type
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Shows the current tty type for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/line
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Shows the current tty line number for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/port
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Shows the current tty port I/O address for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/irq
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Shows the current primary interrupt for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/flags
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Show the tty port status flags for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/xmit_fifo_size
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Show the transmit FIFO size for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/close_delay
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		Show the closing delay time for this port in centiseconds.
+
+		These sysfs values expose the TIOCGSERIAL interface via
+		sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/closing_wait
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		Show the close wait time for this port in centiseconds.
+
+		Waiting forever is represented as 0. If waiting on close is
+		disabled then the value is 65535.
+
+		These sysfs values expose the TIOCGSERIAL interface via
+		sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/custom_divisor
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Show the custom divisor if any that is set on this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/io_type
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Show the I/O type that is to be used with the iomem base
+		 address.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/iomem_base
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 The I/O memory base for this port.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/iomem_reg_shift
+Date:		October 2012
+Contact:	Alan Cox <alan@linux.intel.com>
+Description:
+		 Show the register shift indicating the spacing to be used
+		 for accesses on this iomem address.
+
+		 These sysfs values expose the TIOCGSERIAL interface via
+		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS<x>/rx_trig_bytes
+Date:		May 2014
+Contact:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
+Description:
+		 Shows current RX interrupt trigger bytes or sets the
+		 user specified value to change it for the FIFO buffer.
+		 Users can show or set this value regardless of opening the
+		 serial device file or not.
+
+		 The RX trigger can be set one of four kinds of values for UART
+		 serials. When users input a meaning less value to this I/F,
+		 the RX trigger is changed to the nearest lower value for the
+		 device specification. For example, when user sets 7bytes on
+		 16550A, which has 1/4/8/14 bytes trigger, the RX trigger is
+		 automatically changed to 4 bytes.
+
+What:		/sys/class/tty/ttyS<x>/console
+Date:		February 2020
+Contact:	Andy Shevchenko <andriy.shevchenko@linux.intel.com>
+Description:
+		 Allows user to detach or attach back the given device as
+		 kernel console. It shows and accepts a boolean variable.