[PATCH v3 1/3] posix_timers: do not account group_exec_runtime for dying autoreaped tasks

[Olivier Langlois <olivier@olivierlanglois.net>]

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().

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