When rt_mutex_setprio changes a task's scheduling class to RT, sometimes the task's vruntime is not updated correctly upon return to the fair class. Specifically, the following is being observed: - task has just been created and running for a short time - task sleep while still in the fair class - task is boosted to RT via rt_mutex_setprio, which changes the task to RT and calls check_class_changed. - check_class_changed leads to detach_task_cfs_rq, at which point the vruntime_normalized check sees that the task's sum_exec_runtime is zero, which results in skipping the subtraction of the rq's min_vruntime from the task's vruntime - later, when the prio is deboosted and the task is moved back to the fair class, the fair rq's min_vruntime is added to the task's vruntime, even though it wasn't subtracted earlier. Since the task's vruntime is about double that of other tasks in cfs_rq, the task to be unable to run for a long time when there are continuous runnable tasks in cfs_rq. The immediate result is inflation of the task's vruntime, giving it lower priority (starving it if there's enough available work). The longer-term effect is inflation of all vruntimes because the task's vruntime becomes the rq's min_vruntime when the higher priority tasks go idle. That leads to a vicious cycle, where the vruntime inflation repeatedly doubled. The root cause of the problem is that the vruntime_normalized made a misjudgment. Since the sum_exec_runtime of some tasks that were just created and run for a short time is zero, the vruntime_normalized mistakenly thinks that they are tasks that have just been forked. Therefore, sum_exec_runtime is not subtracted from the vruntime of the task. So, we fix this bug by adding a check condition for newly forked task. Signed-off-by: mingyang.cui <mingyang.cui@xxxxxxxxxx> --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 73a89fbd81be..3d0c14f3731f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11112,7 +11112,7 @@ static inline bool vruntime_normalized(struct task_struct *p) * - A task which has been woken up by try_to_wake_up() and * waiting for actually being woken up by sched_ttwu_pending(). */ - if (!se->sum_exec_runtime || + if (!se->sum_exec_runtime && p->state == TASK_NEW || (p->state == TASK_WAKING && p->sched_remote_wakeup)) return true; -- 2.34.1
