On Tue, 25 May 2021 at 16:30, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote: > > On Tue, 25 May 2021 at 12:34, Odin Ugedal <odin@xxxxxxx> wrote: > > > > Hi, > > > > tir. 25. mai 2021 kl. 11:58 skrev Vincent Guittot <vincent.guittot@xxxxxxxxxx>: > > > Could you give more details about how cfs_rq->avg.load_avg = 4 but > > > cfs_rq->avg.load_sum = 0 ? > > > > > > > cfs_rq->avg.load_sum is decayed and can become null when crossing > > > period which implies an update of cfs_rq->avg.load_avg. This means > > > that your case is generated by something outside the pelt formula ... > > > like maybe the propagation of load in the tree. If this is the case, > > > we should find the error and fix it > > > > Ahh, yeah, that could probably be described better. > > > > It is (as far as I have found out) because the pelt divider is changed, > > and the output from "get_pelt_divider(&cfs_rq->avg)" is changed, resulting > > in a different value being removed than added. > > ok so IIUC, it happens during the adding/removing/propagating > entities' load in the cfs_rq. > > > > > Inside pelt itself, this cannot happen. When pelt changes the load_sum, it > > recalculates the load_avg based on load_sum, and not the delta, afaik. > > > > And as you say, the "issue" therefore (as I see it) outside of PELT. Due to > > how the pelt divider is changed, I assume it is hard to pinpoint where the issue > > is. I can try to find a clear path where where we can see what is added > > and what is removed from both cfs_rq->avg.load_sum and cfs_rq->avg.load_avg, > > to better be able to pinpoint what is happening. > > > > Previously I thought this was a result of precision loss due to division and > > multiplication during load add/remove inside fair.c, but I am not sure that > > is the issue, or is it? > > I don't think the precision looss is the problem because > adding/removing load in fair.c could truncate load_sum but it stays > sync with load_avg. I will have a llo to see if i can see something > weird I have added a trace in cfs_rq_is_decayed() but I'm not able to reproduce a situation where load_sum == 0 but not load_avg even with the script in the cover letter > > > > > If my above line of thought makes sense, do you still view this as an error > > outside PELT, or do you see another possible/better solution? > > > > Will investigate further. > > Thanks > > > > > Thanks > > Odin
