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The timer executes on a housekeeping core and > > > > eventually calls tick_nohz_dep_set_cpu(), sending IPIs to isolated cores > > > > even when only a single task is running. > > > > > > > > The problem is that dl-servers are not coordinated with nohz_full tick > > > > state. Timers can fire and send IPIs to otherwise undisturbed cores. > > > > > > > > Fix by managing servers in sched_can_stop_tick(): > > > > > > > > - When RT tasks run with CFS/SCX tasks, start the appropriate server > > > > and keep the tick running > > > > - When only RT tasks remain, stop all servers and allow tick to stop > > > > (except for >1 RR tasks which need the tick for round-robin) > > > > - When only CFS/SCX tasks remain, stop all servers before stopping tick > > > > > > > > Introduce dl_servers_stop_all() to reduce duplication and abstract > > > > server management from core.c. Unify RT handling into one block that > > > > handles both RR and FIFO cases. > > > > > > > > Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server") > > > > Reported-by: David Haufe > > > > Closes: https://lore.kernel.org/lkml/CAKJHwtOw_G67edzuHVtL1xC5Vyt6StcZzihtDd0yaKudW=rwVw@mail.gmail.com > > > > Signed-off-by: Juri Lelli > > > > --- > > > > I had to modify my first original attempt at fixing this (please take a > > > > look at the linked report/discussion) to also take SCX into > > > > consideration. > > > > > > As mentioned by Frederic, we don't allow to load BPF schedulers when isolcpus= > > > is used, so I think we can simplify the sched_can_stop_tick() part. > > > > Right! Thanks for confirming. > > Ah, but wait. IIUC SCX is incopatible with isolcpus=domain only? > scx_can_stop_tick() seems to confirm we need to take care of it when > domain flag is not present. > > So, maybe we still need to consider SCX in this patch? e.g. in > configurations that are not using static domain isolation, but isolate > CPUs by configuring tasks affinities. Ah! That's right. SCX is incompatible with isolcpus=domain, but we do support nohz_full=..., so I think your original approach is correct. It might be worth calling out explicitly in the patch description that the SCX handling targets nohz_full, so we don't make the same mistake in the future. Thanks, -Andrea