Re: [RFC PATCH] sched/fair: update the vruntime to be max vruntime when yield

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 22, 2023 at 04:03:14PM +0800, Xuewen Yan wrote:
> When task call the sched_yield, cfs would set the cfs's skip buddy.
> If there is no other task call the sched_yield syscall, the task would
> always be skiped when there are tasks in rq. 

So you have two tasks A) which does sched_yield() and becomes ->skip,
and B) which is while(1). And you're saying that once A does it's thing,
B runs forever and starves A?

> As a result, the task's
> vruntime would not be updated for long time, and the cfs's min_vruntime
> is almost not updated.

But the condition in pick_next_entity() should ensure that we still pick
->skip when it becomes too old. Specifically, when it gets more than
wakeup_gran() behind.

> When this scenario happens, when the yield task had wait for a long time,
> and other tasks run a long time, once there is other task call the sched_yield,
> the cfs's skip_buddy is covered, at this time, the first task can run normally,
> but the task's vruntime is small, as a result, the task would always run,
> because other task's vruntime is big. This would lead to other tasks can not
> run for a long time.

I'm not seeing how this could happen, it should never get behind that
far.

Additionally, check_preempt_tick() will explicitly clear the buddies
when it finds the current task has consumed it's ideal slice.

I really cannot see how your scenario can happen.

> In order to mitigate this, update the yield_task's vruntime to be cfs's max vruntime.
> This way, the cfs's min vruntime can be updated as the process running.

This is a bad solution, SCHED_IDLE tasks have very low weight and can be
shot really far to the right, leading to other trouble.