Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 25 Feb 2022 09:29:41 +0100 | Subject | Re: [RFC PATCH 0/5] introduce sched-idle balancing |
| |
On Fri, 25 Feb 2022 at 07:46, Abel Wu <wuyun.abel@bytedance.com> wrote: > > Hi Peter, > > On 2/24/22 11:20 PM, Peter Zijlstra Wrote: > > On Thu, Feb 17, 2022 at 11:43:56PM +0800, Abel Wu wrote: > >> Current load balancing is mainly based on cpu capacity > >> and task util, which makes sense in the POV of overall > >> throughput. While there still might be some improvement > >> can be done by reducing number of overloaded cfs rqs if > >> sched-idle or idle rq exists. > > > > I'm much confused, there is an explicit new-idle balancer and a periodic > > idle balancer already there. > > The two balancers are triggered on the rqs that have no tasks on them, > and load_balance() seems don't show a preference for non-idle tasks so
The load balance will happen at the idle pace if a sched_idle task is running on the cpu so you will have an ILB on each cpu that run a sched-idle task
> there might be possibility that only idle tasks are pulled during load > balance while overloaded rqs (rq->cfs.h_nr_running > 1) exist. As a
There is a LB_MIN feature (disable by default) that filters task with very low load ( < 16) which includes sched-idle task which has a max load of 3
> result the normal tasks, mostly latency-critical ones in our case, on > that overloaded rq still suffer waiting for each other. I observed this > through perf sched. > > IOW the main difference from the POV of load_balance() between the > latency-critical tasks and the idle ones is load. > > The sched-idle balancer is triggered on the sched-idle rqs periodically > and the newly-idle ones. It does a 'fast' pull of non-idle tasks from > the overloaded rqs to the sched-idle/idle ones to let the non-idle tasks > make full use of cpu resources. > > The sched-idle balancer only focuses on non-idle tasks' performance, so > it can introduce overall load imbalance, and that's why I put it before > load_balance().
According to the very low weight of a sched-idle task, I don't expect much imbalance because of sched-idle tasks. But this also depends of the number of sched-idle task.
> > Best Regards, > Abel
| |