On 06/11/25 18:09, Andrea Righi wrote:
On Thu, Nov 06, 2025 at 10:49:20AM +0100, Juri Lelli wrote:
Hi,
On 29/10/25 20:08, Andrea Righi wrote:
During switching from sched_ext to fair tasks and vice-versa, we need support for intializing and removing the bandwidth contribution of either DL server.
My first and more general/design question is do we strictly need this automagic bandwidth management. We seem to agree [1] that we want to move towards explicit dl-server(s) and tasks bandwidth handling, so we might want to consider leaving the burden completely to whomever might be configuring the system.
I think we decided to take this approach because, once a sched_ext scheduler is loaded and all tasks are moved to the ext class, the fair class becomes "empty", but the fair dl-server would still keep its bandwidth reserved, so somehow we need to release that reservation, right?
Right. I was just alluding to the fact that keeping the "empty" fair_server reservations is not wrong, but indeed sub-optimal. I didn't want to block this series if we don't get the automagical removal right, so wondered if it could be left for later (as we will still have a manual way to remove the empty reservations anyway :).
...
hrtimer_cancel(&dl_se->inactive_timer);I am not sure we actually need to force cancel the timer (but still contradicting myself every time I go back at staring at code :). The way I believe this should work 'in theory' is
- we remove a server (either automagic or user sets runtime to 0 - which is probably to fix/look at in current implementation as well btw)
- current bandwidth is retained and only freed (and server reset) at 0-lag (when inactive_timer fires)
- if server is activated back before 0-lag it will use it's current parameters
- after 0-lag it's a new instance with new parameters
Hm... that means just setting the runtime to 0 IIUC. I think I tried that approach in the past, but I was seeing some inconsistencies with the total_bw kselftest, starting/stopping an scx scheduler multiple times seemed to gradually consume all the available bandwidth.
But I can give it another try, maybe that behavior was caused by other issues, since we've fixed quite a few things since then.
Or maybe it could be inactive_timer/dl_non_contending handling that still has some problems.
Anyway, I noticed that it is still possible to write runtime values of fair_server while/after scx_server took over. Those values get overridden when scx_server switches off. Guess we want to prevent writes while scx_server has full control?