On 2021/1/25 22:51, Vincent Guittot wrote: > On Mon, 25 Jan 2021 at 15:00, Li, Aubrey <aubrey...@linux.intel.com> wrote: >> >> On 2021/1/25 18:56, Vincent Guittot wrote: >>> On Mon, 25 Jan 2021 at 06:50, Aubrey Li <aubrey...@intel.com> wrote: >>>> >>>> A long-tail load balance cost is observed on the newly idle path, >>>> this is caused by a race window between the first nr_running check >>>> of the busiest runqueue and its nr_running recheck in detach_tasks. >>>> >>>> Before the busiest runqueue is locked, the tasks on the busiest >>>> runqueue could be pulled by other CPUs and nr_running of the busiest >>>> runqueu becomes 1, this causes detach_tasks breaks with LBF_ALL_PINNED >>> >>> We should better detect that when trying to detach task like below >> >> This should be a compromise from my understanding. If we give up load balance >> this time due to the race condition, we do reduce the load balance cost on >> the >> newly idle path, but if there is an imbalance indeed at the same sched_domain > > Redo path is there in case, LB has found an imbalance but it can't > move some loads from this busiest rq to dest rq because of some cpu > affinity. So it tries to fix the imbalance by moving load onto another > rq of the group. In your case, the imbalance has disappeared because > it has already been pulled by another rq so you don't have to try to > find another imbalance. And I would even say you should not in order > to let other level to take a chance to spread the load
Here is one simple case I have seen: 1) CPU_a becomes idle and invoke newly idle balance 2) Group_b is found as the busiest group 3) CPU_b_1 is found as the busiest CPU, nr_running = 5 4) detach_tasks check CPU_b_1's run queue again, nr_running = 1, goto redo 5) Group_b is still found as the busiest group 6) This time CPU_b_2 is found as the busiest CPU, nr_running = 3 7) detach_tasks successfully, 2 tasks moved. If we skipped redo, - CPU_a exit load balance and remain idle - tasks stay on CPU_b_2's runqueue, wait for the next load balancing The two tasks could have been moved to the idle CPU and get executed immediately. > >> level, we have to wait the next softirq entry to handle that imbalance. This >> means the tasks on the second busiest runqueue have to stay longer, which >> could >> introduce tail latency as well. That's why I introduced a variable to control >> the redo loops. I'll send this to the benchmark queue to see if it makes any > > TBH, I don't like multiplying the number of knobs > I see. Thanks, -Aubrey