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kernel: smp: put comment on SMP code

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Adds some comments to the SMP code to, hopefully, make it
a bit more clear to future readers.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
This commit is contained in:
Daniel Leung 2023-11-02 13:23:06 -07:00 • committed by Anas Nashif
commit fe66e35db0
1 changed files with 51 additions and 1 deletions
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52
kernel/smp.c
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@ -10,7 +10,23 @@
#include <kernel_internal.h>
static atomic_t global_lock;
/**
* Flag to tell recently powered up CPU to start
* initialization routine.
*
* 0 to tell powered up CPU to wait.
* 1 to tell powered up CPU to continue initialization.
*/
static atomic_t cpu_start_flag;
/**
* Flag to tell caller that the target CPU is now
* powered up and ready to be initialized.
*
* 0 if target CPU is not yet ready.
* 1 if target CPU has powered up and ready to be initialized.
*/
static atomic_t ready_flag;
/**
@ -100,10 +116,19 @@ static inline FUNC_NORETURN void smp_init_top(void *arg)
struct k_thread dummy_thread;
struct cpu_start_cb *csc = arg;
/* Let start_cpu() know that this CPU has powered up. */
(void)atomic_set(&ready_flag, 1);
/* Wait for the CPU start caller to signal that
* we can start initialization.
*/
wait_for_start_signal(&cpu_start_flag);
/* Initialize the dummy thread struct so that
* the scheduler can schedule actual threads to run.
*/
z_dummy_thread_init(&dummy_thread);
#ifdef CONFIG_SYS_CLOCK_EXISTS
smp_timer_init();
#endif
@ -113,6 +138,7 @@ static inline FUNC_NORETURN void smp_init_top(void *arg)
csc->fn(csc->arg);
}
/* Let scheduler decide what thread to run next. */
z_swap_unlocked();
CODE_UNREACHABLE; /* LCOV_EXCL_LINE */
@ -120,10 +146,21 @@ static inline FUNC_NORETURN void smp_init_top(void *arg)
static void start_cpu(int id, struct cpu_start_cb *csc)
{
/* Initialize various CPU structs related to this CPU. */
z_init_cpu(id);
/* Clear the ready flag so the newly powered up CPU can
* signal that it has powered up.
*/
(void)atomic_clear(&ready_flag);
/* Power up the CPU */
arch_start_cpu(id, z_interrupt_stacks[id], CONFIG_ISR_STACK_SIZE,
smp_init_top, csc);
/* Wait until the newly powered up CPU to signal that
* it has powered up.
*/
while (!atomic_get(&ready_flag)) {
local_delay();
}
@ -136,7 +173,12 @@ void k_smp_cpu_start(int id, smp_init_fn fn, void *arg)
cpu_start_fn.fn = fn;
cpu_start_fn.arg = arg;
(void)atomic_set(&cpu_start_flag, 1); /* async, don't care */
/* We are only starting one CPU so we do not need to synchronize
* across all CPUs using the start_flag. So just set it to 1.
*/
(void)atomic_set(&cpu_start_flag, 1);
/* Start the CPU! */
start_cpu(id, &cpu_start_fn);
k_spin_unlock(&cpu_start_lock, key);
@ -144,13 +186,21 @@ void k_smp_cpu_start(int id, smp_init_fn fn, void *arg)
void z_smp_init(void)
{
/* We are powering up all CPUs and we want to synchronize their
* entry into scheduler. So set the start flag to 0 here.
*/
(void)atomic_clear(&cpu_start_flag);
/* Just start CPUs one by one. */
unsigned int num_cpus = arch_num_cpus();
for (int i = 1; i < num_cpus; i++) {
start_cpu(i, NULL);
}
/* Let loose those CPUs so they can start scheduling
* threads to run.
*/
(void)atomic_set(&cpu_start_flag, 1);
}