um: Determine sleep based on need_resched()

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. Additionally, because disabling IRQs does not
block SIGALRM, it is also necessary to check for any pending timer
alarms. This is a preparation for adding SMP support.

Signed-off-by: Tiwei Bie <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/include/shared/kern_util.h

···
 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);

arch/um/kernel/process.c

···
 	/* 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)

arch/um/os-Linux/internal.h

···
 void check_tmpexec(void);
 
 /*
+ * signal.c
+ */
+int timer_alarm_pending(void);
+
+/*
  * skas/process.c
  */
 void wait_stub_done(int pid);

arch/um/os-Linux/signal.c

···
 #include <um_malloc.h>
 #include <sys/ucontext.h>
 #include <timetravel.h>
+#include "internal.h"
 
 void (*sig_info[NSIG])(int, struct siginfo *, struct uml_pt_regs *, void *mc) = {
 	[SIGTRAP]	= relay_signal,
···
 void timer_set_signal_handler(void)
 {
 	set_handler(SIGALRM);
+}
+
+int timer_alarm_pending(void)
+{
+	return !!(signals_pending & SIGALRM_MASK);
 }
 
 void set_sigstack(void *sig_stack, int size)

arch/um/os-Linux/time.c

···
 #include <kern_util.h>
 #include <os.h>
 #include <string.h>
+#include "internal.h"
 
 static timer_t event_high_res_timer = 0;
 
···
  */
 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)
+	/*
+	 * Because disabling IRQs does not block SIGALRM, it is also
+	 * necessary to check for any pending timer alarms.
+	 */
+	if (!uml_need_resched() && !timer_alarm_pending())
 		sigsuspend(&old);
-	/* either way, restore the signal mask */
+
+	/* Restore the signal mask. */
 	sigprocmask(SIG_UNBLOCK, &set, NULL);
 }