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sched: Optimize dummy thread usage on SMP

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Nicolas Pitre points out that since these thread structs are just
dummies for the context swtiching, they can be presumed to be "write
only" and thus there's no point in having one per CPU, everyone can
share the same one.

The only gotcha is that we never really documented (nor really have a
place to document) that rule, so it's not theoretically impossible for
an architecture to read back what it might have written underneath
arch_switch().  Leave this in a separate commit for bisection
purposes, but the risk seems very low.

Signed-off-by: Andy Ross <andyross@google.com>
This commit is contained in:
Andy Ross 2024-04-19 15:03:09 -07:00 committed by Carles Cufí
parent 9f4d9989c6
commit 20611f13ca
4 changed files with 5 additions and 5 deletions
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2
kernel/include/ksched.h
View file

@ -37,7 +37,7 @@ BUILD_ASSERT(K_LOWEST_APPLICATION_THREAD_PRIO
#define Z_ASSERT_VALID_PRIO(prio, entry_point) __ASSERT((prio) == -1, "")
#endif /* CONFIG_MULTITHREADING */
extern struct k_thread _thread_dummies[CONFIG_MP_MAX_NUM_CPUS];
extern struct k_thread _thread_dummy;
void z_sched_init(void);
void z_move_thread_to_end_of_prio_q(struct k_thread *thread);

2
kernel/init.c
View file

@ -650,7 +650,7 @@ FUNC_NORETURN void z_cstart(void)
LOG_CORE_INIT();
#if defined(CONFIG_MULTITHREADING)
z_dummy_thread_init(&_thread_dummies[0]);
z_dummy_thread_init(&_thread_dummy);
#endif /* CONFIG_MULTITHREADING */
/* do any necessary initialization of static devices */
z_device_state_init();

4
kernel/sched.c
View file

@ -33,7 +33,7 @@ struct k_spinlock _sched_spinlock;
/* Storage to "complete" the context switch from an invalid/incomplete thread
* context (ex: exiting an ISR that aborted _current)
*/
__incoherent struct k_thread _thread_dummies[CONFIG_MP_MAX_NUM_CPUS];
__incoherent struct k_thread _thread_dummy;
static void update_cache(int preempt_ok);
static void halt_thread(struct k_thread *thread, uint8_t new_state);
@ -1363,7 +1363,7 @@ static void halt_thread(struct k_thread *thread, uint8_t new_state)
* code.
*/
if (dummify && !IS_ENABLED(CONFIG_ARCH_POSIX)) {
z_dummy_thread_init(&_thread_dummies[_current_cpu->id]);
z_dummy_thread_init(&_thread_dummy);
}
}
}

2
kernel/smp.c
View file

@ -122,7 +122,7 @@ static inline void smp_init_top(void *arg)
/* Initialize the dummy thread struct so that
* the scheduler can schedule actual threads to run.
*/
z_dummy_thread_init(&_thread_dummies[arch_curr_cpu()->id]);
z_dummy_thread_init(&_thread_dummy);
}
#ifdef CONFIG_SYS_CLOCK_EXISTS