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

From: Olivier Langlois
Date: Mon Apr 29 2013 - 14:04:21 EST

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@xxxxxxxxxxxxxx>
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);

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