From: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Date: Tue, 1 Aug 2023 17:26:48 +0200 Subject: [PATCH] sched/rt: Don't try push tasks if there are none. 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. I have traces where ~30 CPUs request this ~3 times each before it finally ends. This greatly increases X's runtime while X isn't making much progress. Teach rto_next_cpu() to only return CPUs which also have tasks on their runqueue which can be pushed away. This does not reduce the tell_cpu_to_push() invocations (rto_loop_next counter increments) but reduces the amount of issued rto_push_irq_work_func() if nothing can be done. As the result the overloaded CPU is blocked less often. There are still cases where the "same job" is repeated several times (for instance the current CPU needs to resched but didn't yet because the irq-work is repeated a few times and so the old task remains on the CPU) but the majority of request end in tell_cpu_to_push() before an IPI is issued. Reviewed-by: "Steven Rostedt (Google)" <rostedt@goodmis.org> Link: https://lore.kernel.org/r/20230801152648._y603AS_@linutronix.de Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- kernel/sched/rt.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) Index: linux-6.8.2-rt10/kernel/sched/rt.c =================================================================== @ linux-6.8.2-rt10/kernel/sched/rt.c:2197 @ static int rto_next_cpu(struct root_doma rd->rto_cpu = cpu; - if (cpu < nr_cpu_ids) + if (cpu < nr_cpu_ids) { + if (!has_pushable_tasks(cpu_rq(cpu))) + continue; return cpu; + } rd->rto_cpu = -1;