uml: track and make up lost ticks

Alarm delivery could be noticably late in the !CONFIG_NOHZ case because lost
ticks weren't being taken into account.  This is now treated more carefully,
with the time between ticks being calculated and the appropriate number of
ticks delivered to the timekeeping system.

Cc: Nix <nix@esperi.org.uk>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 51 insertions, 4 deletions

diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 3f1694b134cb..5aade6027e40 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -12,6 +12,7 @@
 #include "as-layout.h"
 #include "kern_util.h"
 #include "os.h"
+#include "process.h"
 #include "sysdep/barrier.h"
 #include "sysdep/sigcontext.h"
 #include "user.h"
diff --git a/arch/um/os-Linux/time.c b/arch/um/os-Linux/time.c
index e49280599465..bee98f466d66 100644
--- a/arch/um/os-Linux/time.c
+++ b/arch/um/os-Linux/time.c
@@ -9,7 +9,9 @@
 #include <time.h>
 #include <sys/time.h>
 #include "kern_constants.h"
+#include "kern_util.h"
 #include "os.h"
+#include "process.h"
 #include "user.h"
 
 int set_interval(void)
@@ -58,12 +60,17 @@ static inline long long timeval_to_ns(const struct timeval *tv)
 long long disable_timer(void)
 {
         struct itimerval time = ((struct itimerval) { { 0, 0 }, { 0, 0 } });
+        int remain, max = UM_NSEC_PER_SEC / UM_HZ;
 
         if (setitimer(ITIMER_VIRTUAL, &time, &time) < 0)
                 printk(UM_KERN_ERR "disable_timer - setitimer failed, "
                        "errno = %d\n", errno);
 
-        return timeval_to_ns(&time.it_value);
+        remain = timeval_to_ns(&time.it_value);
+        if (remain > max)
+                remain = max;
+
+        return remain;
 }
 
 long long os_nsecs(void)
@@ -79,7 +86,44 @@ static int after_sleep_interval(struct timespec *ts)
 {
         return 0;
 }
+
+static void deliver_alarm(void)
+{
+        alarm_handler(SIGVTALRM, NULL);
+}
+
+static unsigned long long sleep_time(unsigned long long nsecs)
+{
+        return nsecs;
+}
+
 #else
+unsigned long long last_tick;
+unsigned long long skew;
+
+static void deliver_alarm(void)
+{
+        unsigned long long this_tick = os_nsecs();
+        int one_tick = UM_NSEC_PER_SEC / UM_HZ;
+
+        if (last_tick == 0)
+                last_tick = this_tick - one_tick;
+
+        skew += this_tick - last_tick;
+
+        while (skew >= one_tick) {
+                alarm_handler(SIGVTALRM, NULL);
+                skew -= one_tick;
+        }
+
+        last_tick = this_tick;
+}
+
+static unsigned long long sleep_time(unsigned long long nsecs)
+{
+        return nsecs > skew ? nsecs - skew : 0;
+}
+
 static inline long long timespec_to_us(const struct timespec *ts)
 {
         return ((long long) ts->tv_sec * UM_USEC_PER_SEC) +
@@ -102,6 +146,8 @@ static int after_sleep_interval(struct timespec *ts)
          */
         if (start_usecs > usec)
                 start_usecs = usec;
+
+        start_usecs -= skew / UM_NSEC_PER_USEC;
         tv = ((struct timeval) { .tv_sec  = start_usecs / UM_USEC_PER_SEC,
                                  .tv_usec = start_usecs % UM_USEC_PER_SEC });
         interval = ((struct itimerval) { { 0, usec }, tv });
@@ -113,8 +159,6 @@ static int after_sleep_interval(struct timespec *ts)
 }
 #endif
 
-extern void alarm_handler(int sig, struct sigcontext *sc);
-
 void idle_sleep(unsigned long long nsecs)
 {
         struct timespec ts;
@@ -126,10 +170,12 @@ void idle_sleep(unsigned long long nsecs)
          */
         if (nsecs == 0)
                 nsecs = UM_NSEC_PER_SEC / UM_HZ;
+
+        nsecs = sleep_time(nsecs);
         ts = ((struct timespec) { .tv_sec       = nsecs / UM_NSEC_PER_SEC,
                                   .tv_nsec      = nsecs % UM_NSEC_PER_SEC });
 
         if (nanosleep(&ts, &ts) == 0)
-                alarm_handler(SIGVTALRM, NULL);
+                deliver_alarm();
         after_sleep_interval(&ts);
 }