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

[John Stultz]

On Thu, Mar 28, 2024 at 8:21 AM mingyang.cui <mingyang.cui@xxxxxxxxxx> 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
> - 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.

Just to make sure I am following: since sum_exec_runtime is updated in
update_curr(), if the task goes to sleep, shouldn't it be dequeued and
thus update_curr() would have been run (and thus sum_exec_runtime
would be non-zero)?

Maybe in this analysis is the new task being boosted while it is still
newly running (instead of sleeping)?

> 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.

This is an interesting find! I'm curious how you detected the problem,
as it might make a good correctness test (which I'm selfishly looking
for more of myself :)

> 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;

This looks like it was applied against an older tree? The p->state
accesses should be under a READ_ONCE (and likely consolidated before
the conditional?)

Also, I wonder if alternatively a call to update_curr() if
(cfs_rq->curr == se) in switched_from_fair() would be good (ie:
normalize it on the boost to close the edge case rather than handle it
as an expected non-normalized condition)?

thanks
-john