Hi, This is RFC v2 of Peter's SCHED_DEADLINE server infrastructure implementation [1]. SCHED_DEADLINE servers can help fixing starvation issues of low priority tasks (e.g., SCHED_OTHER) when higher priority tasks monopolize CPU cycles. Today we have RT Throttling; DEADLINE servers should be able to replace and improve that. I rebased Peter's patches (adding changelogs where needed) on tip/sched/core as of today and incorporated fixes to issues discussed during RFC v1. Current set seems to even boot on real HW! :-) While playing with RFC v1 set (and discussing it further offline with Daniel) it has emerged the need to slightly change the behavior. Patch 6/6 is a (cumbersome?) attempt to show what's probably needed. The problem with "original" implementation is that FIFO tasks might suffer preemption from NORMAL even when spare CPU cycles are available. In fact, fair deadline server is enqueued right away when NORMAL tasks wake up and they are first scheduled by the server, thus potentially preempting a well behaving FIFO task. This is of course not ideal. So, in patch 6/6 I propose to use some kind of starvation monitor/ watchdog that delays enqueuing of deadline servers to the point when fair tasks might start to actually suffer from starvation (just randomly picked HZ/2 for now). One problem I already see with the current implementation is that it adds overhead to fair paths, so I'm pretty sure there are better ways to implement the idea (e.g., Daniel already suggested using a starvation monitor kthread sort of thing). Receiving comments and suggestions is the sole purpose of this posting at this stage. Hopefully we can further discuss the idea at Plumbers in a few weeks. So, please don't focus too much into actual implementation (which I plan to revise anyway after I'm back from pto :), but try to see if this might actually fly. The feature seems to be very much needed. Thanks! Juri 1 - https://lore.kernel.org/lkml/20190726145409.947503076@xxxxxxxxxxxxx/ Juri Lelli (1): sched/fair: Implement starvation monitor Peter Zijlstra (5): sched: Unify runtime accounting across classes sched/deadline: Collect sched_dl_entity initialization sched/deadline: Move bandwidth accounting into {en,de}queue_dl_entity sched/deadline: Introduce deadline servers sched/fair: Add trivial fair server include/linux/sched.h | 28 ++- kernel/sched/core.c | 23 +- kernel/sched/deadline.c | 483 ++++++++++++++++++++++++--------------- kernel/sched/fair.c | 136 ++++++++++- kernel/sched/rt.c | 17 +- kernel/sched/sched.h | 50 +++- kernel/sched/stop_task.c | 16 +- 7 files changed, 522 insertions(+), 231 deletions(-) -- 2.26.2
