Re: [PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio

[Qais Yousef]

On 04/10/24 16:47, Qais Yousef wrote:
> On 04/10/24 11:13, Vincent Guittot wrote:
> 
> > > > Without cgroup, the solution could be straightforward but android uses
> > > > extensively cgroup AFAICT and update_cfs_group() makes impossible to
> > > > track the top cfs waiter and its "prio"
> > >
> > > :(
> > >
> > > IIUC the issue is that we can't easily come up with a single number of
> > > 'effective prio' for N level hierarchy and compare it with another M level
> > > hierarchy..
> > 
> > And then how do you apply it on the hierarchy ?
> 
> (I am not disagreeing with you, just trying to state the reasons more
> explicitly)
> 
> I think the application is easy, attach to the leaf cfs_rq? Which IIUC
> correctly what should happen with proxy execution, but by consuming the context
> of the donor directly without having explicitly to move the lock owner.
> 
> Finding out which hierarchy actually has the highest effective share is not
> straightforward I agree. And if we combine a potential operation of something
> that could move any waiting task to a different hierarchy at anytime, this gets
> even more complex.
> 
> I need to go and find more info, but seems Windows has some kind of boost
> mechanism to help the lock owner to release the lock faster. I wonder if
> something like that could help as interim solution. What we could do is move
> the task to root group as a boost with the simple reweight operation proposed
> here applied. As soon as it releases the lock we should restore it.
> 
> From what I heard in Windows this boost happens randomly (don't quote me on
> this). I am not sure could be our trigger mechanism. We sure don't want to do
> this unconditionally otherwise we break fairness.
> 
> Maybe there are easier ways to introduce a simple such boost mechanism..

FWIW, trying to find the top-fair-waiter and that wasn't as trivial. I needed
to refactor a fair bit of the code that expects the top-waiter to be the
leftmost..

And I haven't looked at that temporary boost mechanism for cfs. Maybe I'll try
that if I get a chance. For the time being, I got this bandaid if anybody is
interested in a temporary 'solution'

--->8---