Re: [PATCH] sched/fair: Fix forked task check in vruntime_normalized

[Honglei Wang]

On 2024/3/28 14:27, mingyang.cui wrote:
> 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
Hi Mingyang,

Did you do the test on the latest tree? vruntime_normalized was removed 
by e8f331bcc2 (sched/smp: Use lag to simplify cross-runqueue placement).

Thanks,
Honglei

> - 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;
>