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, 200 insertions, 0 deletions

diff --git a/drivers/Makefile b/drivers/Makefile
new file mode 100644
index 000000000..0841ea851
--- /dev/null
+++ b/drivers/Makefile
@@ -0,0 +1,200 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the Linux kernel device drivers.
+#
+# 15 Sep 2000, Christoph Hellwig <hch@infradead.org>
+# Rewritten to use lists instead of if-statements.
+#
+
+obj-y				+= cache/
+obj-y				+= irqchip/
+obj-y				+= bus/
+
+obj-y				+= phy/
+
+# GPIO must come after pinctrl as gpios may need to mux pins etc
+obj-$(CONFIG_PINCTRL)		+= pinctrl/
+obj-$(CONFIG_GPIOLIB)		+= gpio/
+obj-y				+= pwm/
+
+# LEDs must come before PCI, it is needed by NPEM driver
+obj-y				+= leds/
+
+obj-y				+= pci/
+
+obj-$(CONFIG_PARISC)		+= parisc/
+obj-$(CONFIG_RAPIDIO)		+= rapidio/
+obj-y				+= video/
+obj-y				+= idle/
+
+# IPMI must come before ACPI in order to provide IPMI opregion support
+obj-y				+= char/ipmi/
+
+obj-$(CONFIG_ACPI)		+= acpi/
+
+# PnP must come after ACPI since it will eventually need to check if acpi
+# was used and do nothing if so
+obj-$(CONFIG_PNP)		+= pnp/
+obj-y				+= amba/
+
+obj-y				+= clk/
+# Many drivers will want to use DMA so this has to be made available
+# really early.
+obj-$(CONFIG_DMADEVICES)	+= dma/
+
+# SOC specific infrastructure drivers.
+obj-y				+= soc/
+obj-$(CONFIG_PM_GENERIC_DOMAINS)	+= pmdomain/
+
+obj-y				+= virtio/
+obj-$(CONFIG_VDPA)		+= vdpa/
+obj-$(CONFIG_XEN)		+= xen/
+
+# regulators early, since some subsystems rely on them to initialize
+obj-$(CONFIG_REGULATOR)		+= regulator/
+
+# reset controllers early, since gpu drivers might rely on them to initialize
+obj-$(CONFIG_RESET_CONTROLLER)	+= reset/
+
+# tty/ comes before char/ so that the VT console is the boot-time
+# default.
+obj-y				+= tty/
+obj-y				+= char/
+
+# iommu/ comes before gpu as gpu are using iommu controllers
+obj-y				+= iommu/
+
+# gpu/ comes after char for AGP vs DRM startup and after iommu
+obj-y				+= gpu/
+
+obj-$(CONFIG_CONNECTOR)		+= connector/
+
+# i810fb depends on char/agp/
+obj-$(CONFIG_FB_I810)           += video/fbdev/i810/
+
+obj-$(CONFIG_PARPORT)		+= parport/
+obj-y				+= base/ block/ misc/ mfd/ nfc/
+obj-$(CONFIG_LIBNVDIMM)		+= nvdimm/
+obj-y				+= dax/
+obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf/
+obj-$(CONFIG_NUBUS)		+= nubus/
+obj-y				+= cxl/
+obj-y				+= macintosh/
+obj-y				+= scsi/
+obj-y				+= nvme/
+obj-$(CONFIG_ATA)		+= ata/
+obj-$(CONFIG_TARGET_CORE)	+= target/
+obj-$(CONFIG_MTD)		+= mtd/
+obj-$(CONFIG_SPI)		+= spi/
+obj-$(CONFIG_SPMI)		+= spmi/
+obj-$(CONFIG_HSI)		+= hsi/
+obj-$(CONFIG_SLIMBUS)		+= slimbus/
+obj-y				+= net/
+obj-$(CONFIG_ATM)		+= atm/
+obj-$(CONFIG_FUSION)		+= message/
+obj-y				+= firewire/
+obj-$(CONFIG_UIO)		+= uio/
+obj-$(CONFIG_VFIO)		+= vfio/
+obj-y				+= cdrom/
+obj-y				+= auxdisplay/
+obj-$(CONFIG_PCCARD)		+= pcmcia/
+obj-$(CONFIG_DIO)		+= dio/
+obj-$(CONFIG_SBUS)		+= sbus/
+obj-$(CONFIG_ZORRO)		+= zorro/
+obj-$(CONFIG_ATA_OVER_ETH)	+= block/aoe/
+obj-$(CONFIG_TC)		+= tc/
+obj-$(CONFIG_USB_PHY)		+= usb/
+obj-$(CONFIG_USB)		+= usb/
+obj-$(CONFIG_USB_SUPPORT)	+= usb/
+obj-$(CONFIG_PCI)		+= usb/
+obj-$(CONFIG_USB_GADGET)	+= usb/
+obj-$(CONFIG_OF)		+= usb/
+obj-$(CONFIG_SERIO)		+= input/serio/
+obj-$(CONFIG_GAMEPORT)		+= input/gameport/
+obj-$(CONFIG_INPUT)		+= input/
+obj-$(CONFIG_RTC_LIB)		+= rtc/
+obj-y				+= i2c/ i3c/ media/
+obj-$(CONFIG_PPS)		+= pps/
+obj-y				+= ptp/
+obj-$(CONFIG_W1)		+= w1/
+obj-y				+= power/
+obj-$(CONFIG_HWMON)		+= hwmon/
+obj-$(CONFIG_THERMAL)		+= thermal/
+obj-$(CONFIG_WATCHDOG)		+= watchdog/
+obj-$(CONFIG_MD)		+= md/
+obj-$(CONFIG_BT)		+= bluetooth/
+obj-$(CONFIG_ACCESSIBILITY)	+= accessibility/
+obj-$(CONFIG_EDAC)		+= edac/
+obj-$(CONFIG_EISA)		+= eisa/
+obj-$(CONFIG_PM_OPP)		+= opp/
+obj-$(CONFIG_CPU_FREQ)		+= cpufreq/
+obj-$(CONFIG_CPU_IDLE)		+= cpuidle/
+obj-y				+= mmc/
+obj-y				+= ufs/
+obj-$(CONFIG_MEMSTICK)		+= memstick/
+obj-$(CONFIG_INFINIBAND)	+= infiniband/
+obj-y				+= firmware/
+obj-$(CONFIG_FWCTL)		+= fwctl/
+obj-$(CONFIG_CRYPTO)		+= crypto/
+obj-$(CONFIG_SUPERH)		+= sh/
+obj-y				+= clocksource/
+obj-$(CONFIG_DCA)		+= dca/
+obj-$(CONFIG_HID_SUPPORT)	+= hid/
+obj-$(CONFIG_PPC_PS3)		+= ps3/
+obj-$(CONFIG_OF)		+= of/
+obj-$(CONFIG_SSB)		+= ssb/
+obj-$(CONFIG_BCMA)		+= bcma/
+obj-$(CONFIG_VHOST_RING)	+= vhost/
+obj-$(CONFIG_VHOST_IOTLB)	+= vhost/
+obj-$(CONFIG_VHOST)		+= vhost/
+obj-$(CONFIG_GREYBUS)		+= greybus/
+obj-$(CONFIG_COMEDI)		+= comedi/
+obj-$(CONFIG_GPIB)		+= gpib/
+obj-$(CONFIG_STAGING)		+= staging/
+obj-y				+= platform/
+
+obj-$(CONFIG_MAILBOX)		+= mailbox/
+obj-$(CONFIG_HWSPINLOCK)	+= hwspinlock/
+obj-$(CONFIG_REMOTEPROC)	+= remoteproc/
+obj-$(CONFIG_RPMSG)		+= rpmsg/
+obj-$(CONFIG_SOUNDWIRE)		+= soundwire/
+
+# Virtualization drivers
+obj-y				+= virt/
+obj-$(CONFIG_HYPERV)		+= hv/
+
+obj-$(CONFIG_PM_DEVFREQ)	+= devfreq/
+obj-$(CONFIG_EXTCON)		+= extcon/
+obj-$(CONFIG_MEMORY)		+= memory/
+obj-$(CONFIG_IIO)		+= iio/
+obj-$(CONFIG_IPACK_BUS)		+= ipack/
+obj-$(CONFIG_NTB)		+= ntb/
+obj-$(CONFIG_POWERCAP)		+= powercap/
+obj-$(CONFIG_MCB)		+= mcb/
+obj-$(CONFIG_PERF_EVENTS)	+= perf/
+obj-$(CONFIG_RAS)		+= ras/
+obj-$(CONFIG_USB4)		+= thunderbolt/
+obj-$(CONFIG_CORESIGHT)		+= hwtracing/coresight/
+obj-y				+= hwtracing/intel_th/
+obj-$(CONFIG_STM)		+= hwtracing/stm/
+obj-$(CONFIG_HISI_PTT)		+= hwtracing/ptt/
+obj-y				+= android/
+obj-$(CONFIG_NVMEM)		+= nvmem/
+obj-$(CONFIG_FPGA)		+= fpga/
+obj-$(CONFIG_FSI)		+= fsi/
+obj-$(CONFIG_TEE)		+= tee/
+obj-$(CONFIG_MULTIPLEXER)	+= mux/
+obj-$(CONFIG_SIOX)		+= siox/
+obj-$(CONFIG_GNSS)		+= gnss/
+obj-$(CONFIG_INTERCONNECT)	+= interconnect/
+obj-$(CONFIG_COUNTER)		+= counter/
+obj-$(CONFIG_MOST)		+= most/
+obj-$(CONFIG_PECI)		+= peci/
+obj-$(CONFIG_HTE)		+= hte/
+obj-$(CONFIG_DRM_ACCEL)		+= accel/
+obj-$(CONFIG_CDX_BUS)		+= cdx/
+obj-$(CONFIG_DPLL)		+= dpll/
+obj-y				+= resctrl/
+
+obj-$(CONFIG_DIBS)		+= dibs/
+obj-$(CONFIG_S390)		+= s390/