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

[Vincent Guittot]

On Wed, 10 Apr 2024 at 08:59, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
>
> On 04/09/24 14:35, Vincent Guittot wrote:
> > On Tue, 9 Apr 2024 at 08:19, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
> > >
> > > On 04/08/24 12:51, John Stultz wrote:
> > > > On Mon, Apr 8, 2024 at 12:17 AM Vincent Guittot
> > > > <vincent.guittot@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Sun, 7 Apr 2024 at 14:27, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
> > > > > >
> > > > > > On 04/05/24 18:16, Qais Yousef wrote:
> > > > > >
> > > > > > > >
> > > > > > > > All that to say that I think the weight is not applied on purpose.
> > > > > > > > This might work for your particular case but there are more changes to
> > > > > > > > be done if you want to apply prio inheritance between cfs tasks.
> > > > > > > >
> > > > > > > > As an example, what about the impact of cgroup on the actual weight
> > > > > > > > and the inherited priority of a task ? If the owner and the waiter
> > > > > > > > don't belong to the same cgroup their own prio is meaningless... task
> > > > > > > > nice -20 in a group with a weight equal to nice 19 vs a task nice 19
> > > > > > > > in a group with a weight equals to nice -20
> > > > > > >
> > > > > > > That is on my mind actually. But I thought it's a separate problem. That has to
> > > > > > > do with how we calculate the effective priority of the pi_task. And probably
> > > > > > > the sorting order to if we agree we need to revert the above. If that is done
> > > > > >
> > > > > > Thinking more about it the revert is not the right thing to do. We want fair
> > > > > > tasks to stay ordered in FIFO for better fairness and avoid potential
> > > > > > starvation issues. It's just the logic for searching the top_waiter need to be
> > > > > > different. If the top_waiter is fair, then we need to traverse the tree to find
> > > > > > the highest nice value. We probably can keep track of this while adding items
> > > > > > to the tree to avoid the search.
> > > > > >
> > > > > > For cgroup; is it reasonable (loosely speaking) to keep track of pi_cfs_rq and
> > > > > > detach_attach_task_cfs_rq() before the reweight? This seems the most
> > > > > > straightforward solution and will contain the complexity to keeping track of
> > > > > > cfs_rq. But it'll have similar issue to proxy execution where a task that
> > > > > > doesn't belong to the cgroup will consume its share..
> > > > >
> > > > > That's a good point, Would proxy execution be the simplest way to fix all this ?
> > >
> > > Is it? Over 4.5 years ago Unity reported to me about performance inversion
> > > problem and that's when proxy execution work was revived as simplest way to fix
> > > all of this. But still no end in sight from what I see. I was and still think
> > > an interim solution in rt_mutex could help a lot of use cases already without
> > > being too complex. Not as elegant and comprehensive like proxy execution, but
> > > given the impact on both userspace and out of tree kernel hacks are growing
> > > waiting for this to be ready, the cost of waiting is high IMHO.
> > >
> > > FWIW, I already heard several feedbacks that PTHREAD_PRIO_INHERIT does nothing.
> > > I think this reweight issue is more serious problem and likely why I heard this
> > > feedback. I could be underestimating the complexity of the fix though So I'll
> >
> > 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 ?

>
> Does proxy execution fix this problem then? If we can't find the top waiter,

I haven't dug enough in the patchset. John ?

> I can't see how proxy execution would work here too. To my understanding it's
> more about how we apply inheritance (by donating execution context of the top
> waiter) instead of manually applying inheritance like we're doing now.
>
> But the logic of finding the top waiter should remain the same, no?
>
> Need to extend my test case to cover this scenario.