Comtrol eCos - Manual (Page 35)

SMP Support
Name
SMP — Support Symmetric Multiprocessing Systems
Description
eCos contains support for limited Symmetric Multi-Processing (SMP). This is only available on selected architec-
tures and platforms. The implementation has a number of restrictions on the kind of hardware supported. These are
described in
the Section called SMP Support in Chapter 9 .
The following sections describe the changes that have been made to the eCos kernel to support SMP operation.
System Startup
The system startup sequence needs to be somewhat different on an SMP system, although this is largely transparent
to application code. The main startup takes place on only one CPU, called the primary CPU. All other CPUs, the
secondary CPUs, are either placed in suspended state at reset, or are captured by the HAL and put into a spin as
they start up. The primary CPU is responsible for copying the DATA segment and zeroing the BSS (if required),
calling HAL variant and platform initialization routines and invoking constructors. It then calls
cyg_start to enter
the application. The application may then create extra threads and other objects.
It is only when the application calls cyg_scheduler_start that the secondary CPUs are initialized. This routine
scans the list of available secondary CPUs and invokes HAL_SMP_CPU_START to start each CPU. Finally it calls an
internal function Cyg_Scheduler::start_cpu to enter the scheduler for the primary CPU.
Each secondary CPU starts in the HAL, where it completes any per-CPU initialization before calling into the kernel
at
cyg_kernel_cpu_startup . Here it claims the scheduler lock and calls Cyg_Scheduler::start_cpu .
Cyg_Scheduler::start_cpu is common to both the primary and secondary CPUs. The first thing this code does
is to install an interrupt object for this CPU’s inter-CPU interrupt. From this point on the code is the same as for
the single CPU case: an initial thread is chosen and entered.
From this point on the CPUs are all equal, eCos makes no further distinction between the primary and secondary
CPUs. However, the hardware may still distinguish between them as far as interrupt delivery is concerned.
Scheduling
To function correctly an operating system kernel must protect its vital data structures, such as the run queues,
from concurrent access. In a single CPU system the only concurrent activities to worry about are asynchronous
interrupts. The kernel can easily guard its data structures against these by disabling interrupts. However, in a multi-
CPU system, this is inadequate since it does not block access by other CPUs.
The eCos kernel protects its vital data structures using the scheduler lock. In single CPU systems this is a simple
counter that is atomically incremented to acquire the lock and decremented to release it. If the lock is decremented
to zero then the scheduler may be invoked to choose a different thread to run. Because interrupts may continue to
be serviced while the scheduler lock is claimed, ISRs are not allowed to access kernel data structures, or call kernel
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