- any cpu can use smp_schedule() to tell another cpu to reschedule
- if an AP is idle, it turns off timer as there is nothing to
preempt, no need to wakeup just to go back to sleep again
- if a cpu makes a process runnable on an idle cpu, it must wake it up
to reschedule
- sys_schedule can change only selected values, -1 means that the
current value should be kept unchanged. For instance we mostly want
to change the scheduling quantum and priority but we want to keep
the process at the current cpu
- RS can hand off its processes to scheduler
- service can read the destination cpu from system.conf
- RS can pass the information farther
- pressing 'B' on the serial cnsole prints statistics for BKL per cpu.
- 'b' resets the counters
- it presents number of cycles each CPU spends in kernel, how many
cycyles it spends spinning while waiting for the BKL
- it shows optimistic estimation in how many cases we get the lock
immediately without spinning. As the test is not atomic the lock may
be already held by some other cpu before we actually try to acquire
it.
- cross-address space copies use these slots to map user memory for
kernel. This avoid any collisions between CPUs
- well, we only have a single CPU running at a time, this is just to
be safe for the future
- machine information contains the number of cpus and the bsp id
- a dummy SMP scheduler which keeps all system processes on BSP and
all other process on APs. The scheduler remembers how many processes
are assigned to each CPU and always picks the one with the least
processes for a new process.
- apic_send_ipi() to send inter-processor interrupts (IPIs)
- APIC IPI schedule and halt handlers to signal x-cpu that a cpu shold
reschedule or halt
- various little changes to let APs run
- no processes are scheduled at the APs and therefore they are idle
except being interrupted by a timer time to time
- tsc_ctr_switch is made cpu local
- although an x86 specific variable it must be declared globaly as the
cpulocal implementation does not allow otherwise
- each CPU has its own runqueues
- processes on BSP are put on the runqueues later after a switch to
the final stack when cpuid works to avoid special cases
- enqueue() and dequeue() use the run queues of the cpu the process is
assigned to
- pick_proc() uses the local run queues
- printing of per-CPU run queues ('2') on serial console
- APs configure local timers
- while configuring local APIC timer the CPUs fiddle with the interrupt
handlers. As the interrupt table is shared the BSP must not run
- APs wait until BSP turns paging on, it is not possible to safely
execute any code on APs until we can turn paging on as well as it
must be done synchronously everywhere
- APs turn paging on but do not continue and wait
- to isolate execution inside kernel we use a big kernel lock
implemented as a spinlock
- the lock is acquired asap after entering kernel mode and released as
late as possible. Only one CPU as a time can execute the core kernel
code
- measurement son real hw show that the overhead of this lock is close
to 0% of kernel time for the currnet system
- the overhead of this lock may be as high as 45% of kernel time in
virtual machines depending on the ratio between physical CPUs
available and emulated CPUs. The performance degradation is
significant
- kernel detects CPUs by searching ACPI tables for local apic nodes
- each CPU has its own TSS that points to its own stack. All cpus boot
on the same boot stack (in sequence) but switch to its private stack
as soon as they can.
- final booting code in main() placed in bsp_finish_booting() which is
executed only after the BSP switches to its final stack
- apic functions to send startup interrupts
- assembler functions to handle CPU features not needed for single cpu
mode like memory barries, HT detection etc.
- new files kernel/smp.[ch], kernel/arch/i386/arch_smp.c and
kernel/arch/i386/include/arch_smp.h
- 16-bit trampoline code for the APs. It is executed by each AP after
receiving startup IPIs it brings up the CPUs to 32bit mode and let
them spin in an infinite loop so they don't do any damage.
- implementation of kernel spinlock
- CONFIG_SMP and CONFIG_MAX_CPUS set by the build system
- most global variables carry information which is specific to the
local CPU and each CPU must have its own copy
- cpu local variable must be declared in cpulocal.h between
DECLARE_CPULOCAL_START and DECLARE_CPULOCAL_END markers using
DECLARE_CPULOCAL macro
- to access the cpu local data the provided macros must be used
get_cpu_var(cpu, name)
get_cpu_var_ptr(cpu, name)
get_cpulocal_var(name)
get_cpulocal_var_ptr(name)
- using this macros makes future changes in the implementation
possible
- switching to ELF will make the declaration of cpu local data much
simpler, e.g.
CPULOCAL int blah;
anywhere in the kernel source code
- kernel turns on IO APICs if no_apic is _not_ set or is equal 0
- pci driver must use the acpi driver to setup IRQ routing otherwise
the system cannot work correctly except systems like KVM that use
only legacy (E)ISA IRQs 0-15
