Messages in this thread | | | Date | Wed, 10 Apr 2024 19:15:37 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio |
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On 04/10/24 10:30, John Stultz wrote: > On Tue, Apr 9, 2024 at 11:59 PM Qais Yousef <qyousef@layalina.io> wrote: > > > > On 04/09/24 14:35, Vincent Guittot wrote: > > > On Tue, 9 Apr 2024 at 08:19, Qais Yousef <qyousef@layalina.io> wrote: > > > > > > > > On 04/08/24 12:51, John Stultz wrote: > > > > > On Mon, Apr 8, 2024 at 12:17 AM Vincent Guittot > > > > > <vincent.guittot@linaro.org> wrote: > > > > > > > > > > > > On Sun, 7 Apr 2024 at 14:27, Qais Yousef <qyousef@layalina.io> 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.. > > > > Does proxy execution fix this problem then? If we can't find the top waiter, > > 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. > > So, while proxy provides a sort of generalized inheritance, it isn't > deep enough in the class scheduler logic to need to really think about > priority/cgroups. > > It just looks at what gets selected to run. That's the most important > task at that moment. It doesn't really need to care about how/why, > that's left to pick_next_task(). > > Since it leaves mutex blocked tasks on the RQ, it allows the class > scheduler logic to pick the most important task (mutex-blocked or not) > to run. Then if a mutex-blocked task gets selected, we will then find > the mutex owner and run it instead so it can release the lock. When > locks are released, if the owner has a "donor" task, the lock is > handed off to the donor. So, this basically uses the > pick_next_task()'s evaluation of what it wanted to run to effectively > provide the "top waiter".
Thanks John. So there's no top waiter and all tasks are left runnable, makes sense.
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