Re: [PATCH 06/20 v3] sched/eevdf: Add min slice check when selecting CPU

[Kayra Cizmeci]

On Mon, Nov 09, 2026 at 04:55:23PM +0100, Vincent Guittot wrote:
> On Sat, 7 Nov 2026 at 09:57, Kayra Cizmeci <kayracizmeci@xxxxxxxxx> wrote:
> >
> >
> > Hi Vincent,
> >
> >
> > Sashiko reviewed this one so I'll just put here my own concerns instead
> > of repeating.
> 
> Regarding sashiko's feedback about cpu state ranking, I chose this
> ranking on purpose.
> For !SMT :
> - 7 CPU is idle and fully fits (i.e. return immediately)
> - 6 CPU fully fits and is not idle but we can preempt current because
> of slice so the task will run on the best CPU immediately
> - 5 CPU is idle but doesn't fit uclamp min. On such a CPU, the task
> can easily miss its deadline
> - 4 CPU is idle but doesn't fit at all. This is even worse
> 
> For SMT
> - 7:CPU is idle and fully fits (i.e. return immediately)
> - 6: CPU is busy and fully fits. Although CPU is not idle, task can
> preempt current because of its slice and run immediately on a CPU.
> This is similar to an idle CPU (and maybe even better). While writing
> this, I realize that I should probably check the lag too
> - 5: CPU is idle but doesn't fit uclamp min. On such a CPU, the task
> can easily miss its deadline
> - 4: CPU is idle but doesn't fit at all. This is even worse than -5
> - 3: CPU is idle and fits for task but sibling is busy
> - 2: CPU and sibling are busy and fits for task and like-6 task will
> run immediately
> - 1: CPU is idle and doesn't fit uclamp and sibling is busy
>   0: CPU is idle but doesn't fit at all, and sibling is busy.
> 
> Note that with the push mecanism we could look for another CPU for the
> task that will be preempted
> 

Thanks for the great explanation! My only concern is on 6 and 2, as I
explained in my first message, below.

Sashiko also pointed out other concerns, it would be good to check them
all out.

> >
> >
> > Look here:
> >
> > CPU1: CPU2:
> > FA    RA
> >       FC
> >
> > CPU2 has 1 RT and 1 Fair tasks named RA and FC.
> > Let's say RA is running on CPU2 and FC's slice is 10 ms.
> >
> > And let's say FA has 8 ms slice, and a fair task named TC is waking up
> > with 9 ms slice:
> >
> > * We look to CPU1, 9 < 8 is false so we return false
> >
> > * We look to CPU2, 9 < 10 is true so we return it.
> >
> > But the problem is, CPU2 has a RT task running and we are going to
> > compete with the RT task. In that case it can be worse than CPU1.
> >


Thanks,

Kayra