“Linux Kernel CPU Hotplug in the Multicore System”
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The respective talk was held by Oleksandr Shevchenko (Senior Engineering Consultant, GlobalLogic) at GlobalLogic Lviv Embedded TechTalk #2 on May 23, 2018. Oleksandr presentation is about features of software architecture, which provides parallel work of Linux and operating system real-time on different cores of a single processor. The talk is also about the Linux mechanism, which allows to connect the processor cores after the boot process has finished, the so-called "CPU Hotplug".
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“Linux Kernel CPU Hotplug in the Multicore System”
- 1. CPU hotplug implementation for the multicore system running RTOS and Linux simultaneously on separate cores Oleksandr Shevchenko Senior Consultant, Engineering
- 2. Agenda 1. Running RTOS and Linux simultaneously on separate cores 2. CPU hotplug implementation 3. Questions?
- 3. 3 Part 1 Running RTOS and Linux simultaneously on separate cores
- 4. Kernel booting Boot Sequence NOR Flash RTOS Linux Kernel (zImage) RootFS (rootfs.cpio.gz) RAM ATAGS
- 5. Kernel booting Boot Sequence NOR Flash RTOS Linux Kernel (zImage) RootFS (rootfs.cpio.gz) RAM RTOS (core 0) By 1-st stage bootloader ATAGS
- 6. Kernel booting Boot SequenceBoot Sequence NOR Flash Linux Kernel (zImage) RootFS (rootfs.cpio.gz) RAM By RTOS Linux Kernel (zImage) RootFS (rootfs.cpio.gz) ATAGS ATAGS RTOS RTOS (core 0)
- 7. Kernel booting Boot SequenceBoot SequenceBoot Sequence NOR Flash Linux Kernel (zImage) RootFS (rootfs.cpio.gz) RAM Linux Kernel RootFS (rootfs.cpio.gz) RootFS Uncompress and mount by Linux Kernel ATAGS Linux Kernel (zImage) Uncompressed by itself ATAGS RTOS RTOS (core 0) (core 1,2,3)
- 8. Kernel booting Boot SequenceBoot SequenceBoot Sequence NOR Flash Linux Kernel (zImage) RootFS (rootfs.cpio.gz) RAM Linux Kernel RootFS ATAGS User Space Services and applications RTOS RTOS (core 0) (core 1,2,3)
- 9. CPU Communication The mutex registers can only be written when it is unused (recognizable by reading a ‘0’). Mutexes can only be reset by writing a ‘0’. Each ARM core should write an identifier to lock the mutex. • ARM core 0: write ‘1’ • ARM core 1: write ‘2’ • ARM core 2: write ‘3’ • ARM core 3: write ‘4’ RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Linux Kernel (core 1) 0 (Reset)Mutex register:
- 10. CPU Communication The mutex registers can only be written when it is unused (recognizable by reading a ‘0’). Mutexes can only be reset by writing a ‘0’. Each ARM core should write an identifier to lock the mutex. • ARM core 0: write ‘1’ • ARM core 1: write ‘2’ • ARM core 2: write ‘3’ • ARM core 3: write ‘4’ RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Linux Kernel (core 1) 1 (Core 0)Mutex register: Core 0 writes 1 to Mutex Register and enters critical section
- 11. CPU Communication The mutex registers can only be written when it is unused (recognizable by reading a ‘0’). Mutexes can only be reset by writing a ‘0’. Each ARM core should write an identifier to lock the mutex. • ARM core 0: write ‘1’ • ARM core 1: write ‘2’ • ARM core 2: write ‘3’ • ARM core 3: write ‘4’ RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Linux Kernel (core 1) 0 (Reset)Mutex register: Core 0 exits critical section by writing 0 to Mutex Register
- 12. CPU Communication The mutex registers can only be written when it is unused (recognizable by reading a ‘0’). Mutexes can only be reset by writing a ‘0’. Each ARM core should write an identifier to lock the mutex. • ARM core 0: write ‘1’ • ARM core 1: write ‘2’ • ARM core 2: write ‘3’ • ARM core 3: write ‘4’ RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Linux Kernel (core 1) 3 (Core 2)Mutex register: Core 2 writes 3 to Mutex Register and enters critical section
- 13. CPU Communication The mutex registers can only be written when it is unused (recognizable by reading a ‘0’). Mutexes can only be reset by writing a ‘0’. Each ARM core should write an identifier to lock the mutex. • ARM core 0: write ‘1’ • ARM core 1: write ‘2’ • ARM core 2: write ‘3’ • ARM core 3: write ‘4’ RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Linux Kernel (core 1) 0 (Reset)Mutex register: Core 2 exits critical section by writing 0 to Mutex Register
- 14. RTOS/Linux Device Driver Architecture Physical Devices (Hardware) Device Drivers Virtual File System Kernel Space User Space Application Open/ Close Read/ Write IOCTL Linux
- 15. RTOS/Linux Device Driver Architecture Physical Devices (Hardware) Device Drivers Virtual File System Kernel Space User Space Application Open/ Close Read/ Write IOCTL Linux XAPI RTOS Application
- 16. RTOS/Linux Device Driver Architecture Linux Driver file opsApplication fd=open(“/dev/module_xyz”, …); modXyzOpen() Private data handle last_error alloc XAPI Core API XAPI_XYZ_Open() XAPI_XYZ_SetX() XAPI_XYZ_GetY() close(fd) modXyzClose() free XAPI_XYZ_Close() XAPI_XYZ_Read() XAPI_XYZ_Write() modXyzIoctl()ioctl(fd, cmd, …) User Space Kernel Space modXyzRead() modXyzWrite() read(fd, …) write(fd, …)
- 17. 17 Part 1 Summary 1. Introduce correct boot sequence.
- 18. 18 Part 1 Summary 1. Introduce correct boot sequence. 2. Provide inter-CPU synchronization mechanism.
- 19. 19 Part 1 Summary 1. Introduce correct boot sequence. 2. Provide inter-CPU synchronization mechanism. 3. Implement common Hardware Abstraction Level (HAL) for RTOS and Linux.
- 20. 20 PART 2 CPU hotplug implementation
- 21. Decrease Response Time on Power On RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) RTOS (core 1) 1. Start 2 samples of RTOS on two cores.
- 22. RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) 1. Start 2 samples of RTOS on two cores. 2. Once time critical task is done – withdraw from one core. RTOS (core 1) Decrease Response Time on Power On
- 23. RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Idle (core 1) 1. Start 2 samples of RTOS on two cores. 2. Once time critical task is done – withdraw from one core. Decrease Response Time on Power On
- 24. RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) Idle (core 1) 1. Start 2 samples of RTOS on two cores. 2. Once time critical task is done – withdraw from one core. 3. Linux Hot-plugs the idle core Decrease Response Time on Power On
- 25. RTOS (core 0) Linux Kernel (core 2) Linux Kernel (core 3) 1. Start 2 samples of RTOS on two cores. 2. Once time critical task is done – withdraw from one core. 3. Linux Hot-plugs the idle core Linux Kernel (core 1) Decrease Response Time on Power On
- 26. Patching Linux Kernel SMP Init Code main.c kernel_init() wakeup_secondary() is a place where the CPU state is changed from IDLE to EXECUTE. smp.c smp_prepare_cpus() arch/arm/mach-fujitsu/ smp-hd62x.c platform_smp_prepare_cpus() wakeup_secondary()
- 27. Patching Linux Kernel SMP Init Code
- 28. Patching Linux Kernel SMP Init Code
- 29. Patching Linux Kernel SMP Init Code main.c kernel_init() __cpu_up() is the place where a new process spawn for the core #cpu with fork_idle(cpu) smp.c smp_init() cpu.c cpu_up() _cpu_up() __cpu_up()
- 30. Patching Linux Kernel SMP Init Code In __cpu_up() added functionality to skip CPU1 initialization during kernel start-up. + static unsigned flag=0; + + if ((cpu == 1) && (flag == 0)) + { + printk(KERN_ERR "CPU%u: Skip initialization...n", cpu); + flag = 1; + return -EBUSY; + } We should quit at this point during initialization phase and do not try to spawn a new process for the core #1 with fork_idle(cpu). We will do it later while hot-plugging.
- 31. Patching Linux Kernel SMP Init Code Linux kernel already has built-in CPU core hot-plug functionality. To activate CPU1 core hot-plug the following sysfs write command has to be issued: echo 1 > /sys/devices/system/cpu/cpu1/online But this will only work correctly if the CPU core had been previously initialized during the startup and then disabled by command echo 0 > /sys/devices/system/cpu/cpu1/online
- 32. Patching Linux Kernel SMP Init Code While writing to the cpu subsystem related sysfs file, the store_online() function from src/drivers/base/cpu.c is called by kernel. We added new wakeup_secondary() function to this file +static void wakeup_secondary(void) … + jump_address = virt_to_phys(fujitsu_secondary_startup); + writel(jump_address, BOOT_CPU_CONTROL_CPU1_JUMP); + control_value = __raw_readl(BOOT_CPU_CONTROL_CPU1_STATUS); + control_value &= ~BOOT_CPU_CONTROL_REQUEST_MASK; + control_value &= ~BOOT_CPU_CONTROL_STATUS_MASK; + control_value |= BOOT_CPU_CONTROL_REQUEST_EXECUTE; + writel(control_value, BOOT_CPU_CONTROL_CPU1_STATUS); We call this function before cpu_up() function that brings the CPU core to Linux world. case '1': + wakeup_secondary(); ret = cpu_up(cpu->sysdev.id);
- 33. Patching Linux Kernel SMP Init Code And of course before hot-plugging from Linux the RTOS should withdraw from the core 1 and send it to POLLING state by writing correct value to the CPU control register BOOT_CPU_CONTROL_CPU1_STATUS
- 34. Check if it works 1. Run Linux console command: # cat /sys/devices/system/cpu/online 0-2 2. Run CPU3 hot-plug command from Linux console: # echo 1 > /sys/devices/system/cpu/cpu3/online CPU3: Booted secondary processor CPU3: thread -1, cpu 3, socket 0, mpidr 80000003 HD62: Set local_timer Event Freq: 198000kHz Mult: 850403525 Shift: 32 CPU3: Unknown IPI message 0x1Sadf 3. Make sure that Linux is now running on 4 cores: # cat /sys/devices/system/cpu/online 0-3
- 35. Check if it works Check the output of #htop before and after hotplugging.
- 36. 36 Part 2 Summary 1. Modify smp_prepare_cpus() to not put the target CPU core to EXECUTE state.
- 37. 37 Part 2 Summary 1. Modify smp_prepare_cpus() to not put the target CPU core to EXECUTE state. 2. Modify __cpu_up to not spawn a new process for the target core with fork_idle(cpu).
- 38. 38 Part 2 Summary 1. Modify smp_prepare_cpus() to not put the target CPU core to EXECUTE state. 2. Modify __cpu_up to not spawn a new process for the target core with fork_idle(cpu). 3. Modify the RTOS code to put the target CPU core to POLLING state on exit.
- 39. Questions?
- 40. Thank you!