1 files changed, 25 insertions, 8 deletions

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 9858266fb0b3..a846cf89eb96 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -263,6 +263,11 @@ void account_idle_time(cputime_t cputime)
                 cpustat[CPUTIME_IDLE] += (__force u64) cputime;
 }
 
+/*
+ * When a guest is interrupted for a longer amount of time, missed clock
+ * ticks are not redelivered later. Due to that, this function may on
+ * occasion account more time than the calling functions think elapsed.
+ */
 static __always_inline cputime_t steal_account_process_time(cputime_t maxtime)
 {
 #ifdef CONFIG_PARAVIRT
@@ -371,7 +376,7 @@ static void irqtime_account_process_tick(struct task_struct *p, int user_tick,
          * idle, or potentially user or system time. Due to rounding,
          * other time can exceed ticks occasionally.
          */
-        other = account_other_time(cputime);
+        other = account_other_time(ULONG_MAX);
         if (other >= cputime)
                 return;
         cputime -= other;
@@ -486,7 +491,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
         }
 
         cputime = cputime_one_jiffy;
-        steal = steal_account_process_time(cputime);
+        steal = steal_account_process_time(ULONG_MAX);
 
         if (steal >= cputime)
                 return;
@@ -516,7 +521,7 @@ void account_idle_ticks(unsigned long ticks)
         }
 
         cputime = jiffies_to_cputime(ticks);
-        steal = steal_account_process_time(cputime);
+        steal = steal_account_process_time(ULONG_MAX);
 
         if (steal >= cputime)
                 return;
@@ -614,19 +619,25 @@ static void cputime_adjust(struct task_cputime *curr,
         stime = curr->stime;
         utime = curr->utime;
 
-        if (utime == 0) {
-                stime = rtime;
+        /*
+         * If either stime or both stime and utime are 0, assume all runtime is
+         * userspace. Once a task gets some ticks, the monotonicy code at
+         * 'update' will ensure things converge to the observed ratio.
+         */
+        if (stime == 0) {
+                utime = rtime;
                 goto update;
         }
 
-        if (stime == 0) {
-                utime = rtime;
+        if (utime == 0) {
+                stime = rtime;
                 goto update;
         }
 
         stime = scale_stime((__force u64)stime, (__force u64)rtime,
                             (__force u64)(stime + utime));
 
+update:
         /*
          * Make sure stime doesn't go backwards; this preserves monotonicity
          * for utime because rtime is monotonic.
@@ -649,7 +660,6 @@ static void cputime_adjust(struct task_cputime *curr,
                 stime = rtime - utime;
         }
 
-update:
         prev->stime = stime;
         prev->utime = utime;
 out:
@@ -694,6 +704,13 @@ static cputime_t get_vtime_delta(struct task_struct *tsk)
         unsigned long now = READ_ONCE(jiffies);
         cputime_t delta, other;
 
+        /*
+         * Unlike tick based timing, vtime based timing never has lost
+         * ticks, and no need for steal time accounting to make up for
+         * lost ticks. Vtime accounts a rounded version of actual
+         * elapsed time. Limit account_other_time to prevent rounding
+         * errors from causing elapsed vtime to go negative.
+         */
         delta = jiffies_to_cputime(now - tsk->vtime_snap);
         other = account_other_time(delta);
         WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);