[PATCH v2 05/10] um: Determine sleep based on need_resched()

[Tiwei Bie <tiwei.bie@linux.dev>]

From: Tiwei Bie <tiwei.btw@antgroup.com>

With SMP and NO_HZ enabled, the CPU may still need to sleep even
if the timer is disarmed. Switch to deciding whether to sleep based
on pending resched. This is a preparation for adding SMP support.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
---
 arch/um/include/shared/kern_util.h |  1 +
 arch/um/kernel/process.c           | 12 ++++++++++--
 arch/um/os-Linux/time.c            | 11 +++++------
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/um/include/shared/kern_util.h b/arch/um/include/shared/kern_util.h
index 00ca3e12fd9a..3daaa5c4b35d 100644
--- a/arch/um/include/shared/kern_util.h
+++ b/arch/um/include/shared/kern_util.h
@@ -55,6 +55,7 @@ extern int __uml_cant_sleep(void);
 extern int get_current_pid(void);
 extern int copy_from_user_proc(void *to, void *from, int size);
 extern char *uml_strdup(const char *string);
+int uml_need_resched(void);
 
 extern unsigned long to_irq_stack(unsigned long *mask_out);
 extern unsigned long from_irq_stack(int nested);
diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 1be644de9e41..01b935c00454 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -209,10 +209,13 @@ int arch_dup_task_struct(struct task_struct *dst,
 
 void um_idle_sleep(void)
 {
-	if (time_travel_mode != TT_MODE_OFF)
+	if (time_travel_mode != TT_MODE_OFF) {
 		time_travel_sleep();
-	else
+	} else {
+		raw_local_irq_enable();
 		os_idle_sleep();
+		raw_local_irq_disable();
+	}
 }
 
 void arch_cpu_idle(void)
@@ -225,6 +228,11 @@ int __uml_cant_sleep(void) {
 	/* Is in_interrupt() really needed? */
 }
 
+int uml_need_resched(void)
+{
+	return need_resched();
+}
+
 extern exitcall_t __uml_exitcall_begin, __uml_exitcall_end;
 
 void do_uml_exitcalls(void)
diff --git a/arch/um/os-Linux/time.c b/arch/um/os-Linux/time.c
index 4d5591d96d8c..f25a4196bab7 100644
--- a/arch/um/os-Linux/time.c
+++ b/arch/um/os-Linux/time.c
@@ -98,18 +98,17 @@ long long os_nsecs(void)
  */
 void os_idle_sleep(void)
 {
-	struct itimerspec its;
 	sigset_t set, old;
 
-	/* block SIGALRM while we analyze the timer state */
+	/* Block SIGALRM while performing the need_resched check. */
 	sigemptyset(&set);
 	sigaddset(&set, SIGALRM);
 	sigprocmask(SIG_BLOCK, &set, &old);
 
-	/* check the timer, and if it'll fire then wait for it */
-	timer_gettime(event_high_res_timer, &its);
-	if (its.it_value.tv_sec || its.it_value.tv_nsec)
+	/* Sleep if no resched is pending. */
+	if (!uml_need_resched())
 		sigsuspend(&old);
-	/* either way, restore the signal mask */
+
+	/* Restore the signal mask. */
 	sigprocmask(SIG_UNBLOCK, &set, NULL);
 }
-- 
2.34.1