uml: tickless support

Enable tickless support.

CONFIG_TICK_ONESHOT and CONFIG_NO_HZ are enabled.

itimer_clockevent gets CLOCK_EVT_FEAT_ONESHOT and an implementation of
.set_next_event.

CONFIG_UML_REAL_TIME_CLOCK goes away because it only makes sense when there is
a clock ticking away all the time.  timer_handler now just calls do_IRQ once
without trying to figure out how many ticks to emulate.

The idle loop now needs to turn ticking on and off.

Userspace ticks keep happening as usual.  However, the userspace loop keep
track of when the next wakeup should happen and suppresses process ticks until
that happens.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 13 insertions, 35 deletions

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 56d75afedbf7..aef494b6b81a 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -13,6 +13,7 @@
 #include "linux/ptrace.h"
 #include "linux/random.h"
 #include "linux/sched.h"
+#include "linux/tick.h"
 #include "linux/threads.h"
 #include "asm/pgtable.h"
 #include "asm/uaccess.h"
@@ -244,9 +245,11 @@ void default_idle(void)
                 if (need_resched())
                         schedule();
 
+                tick_nohz_stop_sched_tick();
                 switch_timers(1);
                 idle_sleep(10);
                 switch_timers(0);
+                tick_nohz_restart_sched_tick();
         }
 }
 
diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index 3cb7135e5c47..2acdc7efb2ac 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -20,41 +20,12 @@ unsigned long long sched_clock(void)
         return (unsigned long long)jiffies_64 * (1000000000 / HZ);
 }
 
-#ifdef CONFIG_UML_REAL_TIME_CLOCK
-static unsigned long long prev_nsecs[NR_CPUS];
-static long long delta[NR_CPUS];                /* Deviation per interval */
-#endif
-
 void timer_handler(int sig, struct uml_pt_regs *regs)
 {
-        unsigned long long ticks = 0;
         unsigned long flags;
-#ifdef CONFIG_UML_REAL_TIME_CLOCK
-        int c = cpu();
-        if (prev_nsecs[c]) {
-                /* We've had 1 tick */
-                unsigned long long nsecs = os_nsecs();
-
-                delta[c] += nsecs - prev_nsecs[c];
-                prev_nsecs[c] = nsecs;
-
-                /* Protect against the host clock being set backwards */
-                if (delta[c] < 0)
-                        delta[c] = 0;
-
-                ticks += (delta[c] * HZ) / BILLION;
-                delta[c] -= (ticks * BILLION) / HZ;
-        }
-        else prev_nsecs[c] = os_nsecs();
-#else
-        ticks = 1;
-#endif
 
         local_irq_save(flags);
-        while (ticks > 0) {
-                do_IRQ(TIMER_IRQ, regs);
-                ticks--;
-        }
+        do_IRQ(TIMER_IRQ, regs);
         local_irq_restore(flags);
 }
 
@@ -68,10 +39,8 @@ static void itimer_set_mode(enum clock_event_mode mode,
 
         case CLOCK_EVT_MODE_SHUTDOWN:
         case CLOCK_EVT_MODE_UNUSED:
-                disable_timer();
-                break;
         case CLOCK_EVT_MODE_ONESHOT:
-                BUG();
+                disable_timer();
                 break;
 
         case CLOCK_EVT_MODE_RESUME:
@@ -79,13 +48,19 @@ static void itimer_set_mode(enum clock_event_mode mode,
         }
 }
 
+static int itimer_next_event(unsigned long delta,
+                             struct clock_event_device *evt)
+{
+        return timer_one_shot(delta + 1);
+}
+
 static struct clock_event_device itimer_clockevent = {
         .name           = "itimer",
         .rating         = 250,
         .cpumask        = CPU_MASK_ALL,
-        .features       = CLOCK_EVT_FEAT_PERIODIC,
+        .features       = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT,
         .set_mode       = itimer_set_mode,
-        .set_next_event = NULL,
+        .set_next_event = itimer_next_event,
         .shift          = 32,
         .irq            = 0,
 };