1 files changed, 62 insertions, 2 deletions

diff --git a/fs/select.c b/fs/select.c
index f6dceb56793f..5e61b43d0766 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -28,6 +28,58 @@
 
 #include <asm/uaccess.h>
 
+
+/*
+ * Estimate expected accuracy in ns from a timeval.
+ *
+ * After quite a bit of churning around, we've settled on
+ * a simple thing of taking 0.1% of the timeout as the
+ * slack, with a cap of 100 msec.
+ * "nice" tasks get a 0.5% slack instead.
+ *
+ * Consider this comment an open invitation to come up with even
+ * better solutions..
+ */
+
+static unsigned long __estimate_accuracy(struct timespec *tv)
+{
+        unsigned long slack;
+        int divfactor = 1000;
+
+        if (task_nice(current))
+                divfactor = divfactor / 5;
+
+        slack = tv->tv_nsec / divfactor;
+        slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);
+
+        if (slack > 100 * NSEC_PER_MSEC)
+                slack =  100 * NSEC_PER_MSEC;
+        return slack;
+}
+
+static unsigned long estimate_accuracy(struct timespec *tv)
+{
+        unsigned long ret;
+        struct timespec now;
+
+        /*
+         * Realtime tasks get a slack of 0 for obvious reasons.
+         */
+
+        if (current->policy == SCHED_FIFO ||
+                current->policy == SCHED_RR)
+                return 0;
+
+        ktime_get_ts(&now);
+        now = timespec_sub(*tv, now);
+        ret = __estimate_accuracy(&now);
+        if (ret < current->timer_slack_ns)
+                return current->timer_slack_ns;
+        return ret;
+}
+
+
+
 struct poll_table_page {
         struct poll_table_page * next;
         struct poll_table_entry * entry;
@@ -262,6 +314,7 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
         struct poll_wqueues table;
         poll_table *wait;
         int retval, i, timed_out = 0;
+        unsigned long slack = 0;
 
         rcu_read_lock();
         retval = max_select_fd(n, fds);
@@ -278,6 +331,9 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
                 timed_out = 1;
         }
 
+        if (end_time)
+                slack = estimate_accuracy(end_time);
+
         retval = 0;
         for (;;) {
                 unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
@@ -353,7 +409,7 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
                         to = &expire;
                 }
 
-                if (!schedule_hrtimeout(to, HRTIMER_MODE_ABS))
+                if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
                         timed_out = 1;
         }
         __set_current_state(TASK_RUNNING);
@@ -593,6 +649,7 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
         poll_table* pt = &wait->pt;
         ktime_t expire, *to = NULL;
         int timed_out = 0, count = 0;
+        unsigned long slack = 0;
 
         /* Optimise the no-wait case */
         if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
@@ -600,6 +657,9 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
                 timed_out = 1;
         }
 
+        if (end_time)
+                slack = estimate_accuracy(end_time);
+
         for (;;) {
                 struct poll_list *walk;
 
@@ -646,7 +706,7 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
                         to = &expire;
                 }
 
-                if (!schedule_hrtimeout(to, HRTIMER_MODE_ABS))
+                if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
                         timed_out = 1;
         }
         __set_current_state(TASK_RUNNING);