Messages in this thread | | | Date | Thu, 10 Aug 2023 10:07:08 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH] sched/rt: Don't try push tasks if there are none. |
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On 2023-08-09 18:02:32 [+0100], Valentin Schneider wrote: > On 01/08/23 17:26, Sebastian Andrzej Siewior wrote: > > I have a RT task X at a high priority and cyclictest on each CPU with > > lower priority than X's. If X is active and each CPU wakes their own > > cylictest thread then it ends in a longer rto_push storm. > > A random CPU determines via balance_rt() that the CPU on which X is > > running needs to push tasks. X has the highest priority, cyclictest is > > next in line so there is nothing that can be done since the task with > > the higher priority is not touched. > > > > tell_cpu_to_push() increments rto_loop_next and schedules > > rto_push_irq_work_func() on X's CPU. The other CPUs also increment the > > loop counter and do the same. Once rto_push_irq_work_func() is active it > > does nothing because it has _no_ pushable tasks on its runqueue. Then > > checks rto_next_cpu() and decides to queue irq_work on the local CPU > > because another CPU requested a push by incrementing the counter. > > > > For a CPU to be in the rto_mask, it needs: > > rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1 > > But if that CPU has no pushable tasks, then that means only the current > task has p->nr_cpus_allowed > 1. > > Should we change it so a CPU is only in the rto_mask iff it has pushable > tasks? AFAICT that should not break the case where we push the current task > away due to migration_disabled, as that still relies on the > migration_disabled task to be in the pushable list.
Sounds good. The task with the highest priority becomes pushable if it gets preempted (by a task with higher priority). This gets considered, right?
Sebastian
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