[uml-devel] [PATCH 3/3] UML - Track and make up lost ticks

[Jeff Dike <jdike@addtoit.com>]

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>
---
 arch/um/include/process.h |    4 +--
 arch/um/os-Linux/signal.c |    1 
 arch/um/os-Linux/time.c   |   54 ++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 53 insertions(+), 6 deletions(-)

Index: linux-2.6-git/arch/um/os-Linux/time.c
===================================================================
--- linux-2.6-git.orig/arch/um/os-Linux/time.c	2008-05-09 12:18:06.000000000 -0400
+++ linux-2.6-git/arch/um/os-Linux/time.c	2008-05-09 12:30:23.000000000 -0400
@@ -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(co
 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 t
 {
 	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 t
 	 */
 	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 t
 }
 #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);
 }
Index: linux-2.6-git/arch/um/include/process.h
===================================================================
--- linux-2.6-git.orig/arch/um/include/process.h	2008-05-09 12:30:56.000000000 -0400
+++ linux-2.6-git/arch/um/include/process.h	2008-05-09 12:31:04.000000000 -0400
@@ -11,7 +11,7 @@
 /* Copied from linux/compiler-gcc.h since we can't include it directly */
 #define barrier() __asm__ __volatile__("": : :"memory")
 
-extern void sig_handler(int sig, struct sigcontext sc);
-extern void alarm_handler(int sig, struct sigcontext sc);
+extern void sig_handler(int sig, struct sigcontext *sc);
+extern void alarm_handler(int sig, struct sigcontext *sc);
 
 #endif
Index: linux-2.6-git/arch/um/os-Linux/signal.c
===================================================================
--- linux-2.6-git.orig/arch/um/os-Linux/signal.c	2008-05-09 12:21:29.000000000 -0400
+++ linux-2.6-git/arch/um/os-Linux/signal.c	2008-05-09 12:30:29.000000000 -0400
@@ -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"

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