[PATCH v2 1/3] process cputimer is moving faster than its corresponding clock

[Olivier Langlois <olivier@trillion01.com>]

Forbids the cputimer to drift ahead of its process clock by
blocking its update when a tick occurs while a autoreaping task
is currently in do_exit() between the call to release_task() and
its final call to schedule().

Any task stats update after having called release_task() will
be lost because they are added to the global process stats located
in the signal struct from release_task().

Ideally, you should postpone the release_task() call after the
final context switch to get all the stats added but this is
more complex to achieve.

In other words, this is slowing down the cputimer so it keep the same
pace than the process clock but in fact, what should be done is to
speed up the process clock by adding the missing stats to it.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
---
 kernel/sched/fair.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a33e59..52d7b10 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -708,7 +708,15 @@ static void update_curr(struct cfs_rq *cfs_rq)
 
                trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
                cpuacct_charge(curtask, delta_exec);
-               account_group_exec_runtime(curtask, delta_exec);
+               /*
+                * Do not update the cputimer if the task is already released by
+                * release_task().
+                *
+                * it would preferable to defer the autoreap release_task
+                * after the last context switch but harder to do.
+                */
+               if (likely(curtask->sighand))
+                       account_group_exec_runtime(curtask, delta_exec);
        }
 
        account_cfs_rq_runtime(cfs_rq, delta_exec);
-- 
1.8.2.1