sched: Fix clock_gettime(CLOCK_[PROCESS/THREAD]_CPUTIME_ID) monotonicity

If a task has been dequeued, it has been accounted.  Do not project
cycles that may or may not ever be accounted to a dequeued task, as
that may make clock_gettime() both inaccurate and non-monotonic.

Protect update_rq_clock() from slight TSC skew while at it.

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: kosaki.motohiro@jp.fujitsu.com
Cc: pjt@google.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1403588980.29711.11.camel@marge.simpson.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 866d840b99ca..e50234ba0b27 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -139,6 +139,8 @@ void update_rq_clock(struct rq *rq)
                 return;
 
         delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
+        if (delta < 0)
+                return;
         rq->clock += delta;
         update_rq_clock_task(rq, delta);
 }
@@ -2431,7 +2433,12 @@ static u64 do_task_delta_exec(struct task_struct *p, struct rq *rq)
 {
         u64 ns = 0;
 
-        if (task_current(rq, p)) {
+        /*
+         * Must be ->curr _and_ ->on_rq.  If dequeued, we would
+         * project cycles that may never be accounted to this
+         * thread, breaking clock_gettime().
+         */
+        if (task_current(rq, p) && p->on_rq) {
                 update_rq_clock(rq);
                 ns = rq_clock_task(rq) - p->se.exec_start;
                 if ((s64)ns < 0)
@@ -2474,8 +2481,10 @@ unsigned long long task_sched_runtime(struct task_struct *p)
          * If we race with it leaving cpu, we'll take a lock. So we're correct.
          * If we race with it entering cpu, unaccounted time is 0. This is
          * indistinguishable from the read occurring a few cycles earlier.
+         * If we see ->on_cpu without ->on_rq, the task is leaving, and has
+         * been accounted, so we're correct here as well.
          */
-        if (!p->on_cpu)
+        if (!p->on_cpu || !p->on_rq)
                 return p->se.sum_exec_runtime;
 #endif