On 09/19/19 14:27, Vincent Guittot wrote: > > > > But for requirement of performance, I think it is better to > > > > differentiate between idle CPU and CPU has CFS task. > > > > > > > > For example, we use rt-app to evaluate runnable time on non-patched > > > > environment. > > > > There are (NR_CPUS-1) heavy CFS tasks and 1 RT Task. When a CFS task is > > > > running, the RT task wakes up and choose the same CPU. > > > > The CFS task will be preempted and keep runnable until it is migrated > > > > to another cpu by load balance. > > > > But load balance is not triggered immediately, it will be triggered > > > > until timer tick hits with some condition satisfied(ex. > > > > rq->next_balance). > > > > > > Yes you will have to wait for the next tick that will trigger an idle > > > load balance because you have an idle cpu and 2 runnable tack (1 RT + > > > 1CFS) on the same CPU. But you should not wait for more than 1 tick > > > > > > The current load_balance doesn't handle correctly the situation of 1 > > > CFS and 1 RT task on same CPU while 1 CPU is idle. There is a rework > > > of the load_balance that is under review on the mailing list that > > > fixes this problem and your CFS task should migrate to the idle CPU > > > faster than now > > > > > > > Period load balance should be triggered when current jiffies is behind > > rq->next_balance, but rq->next_balance is not often exactly the same > > with next tick. > > If cpu_busy, interval = sd->balance_interval * sd->busy_factor, and > > But if there is an idle CPU on the system, the next idle load balance > should apply shortly because the busy_factor is not used for this CPU > which is not busy. > In this case, the next_balance interval is sd_weight which is probably > 4ms at cluster level and 8ms at system level in your case. This means > between 1 and 2 ticks
But if the CFS task we're preempting was latency sensitive - this 1 or 2 tick is too late of a recovery. So while it's good we recover, but a preventative approach would be useful too. Just saying :-) I'm still not sure if this is the best longer term approach. -- Qais Yousef
