Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/rt: Don't try push tasks if there are none. | Date | Wed, 09 Aug 2023 18:02:32 +0100 |
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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.
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