Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 23 Feb 2021 18:33:26 +0100 | Subject | Re: [RFC PATCH v1] sched/fair: limit load balance redo times at the same sched_domain level |
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On Tue, 23 Feb 2021 at 06:41, Li, Aubrey <aubrey.li@linux.intel.com> wrote: > > Hi Vincent, > > Sorry for the delay, I just returned from Chinese New Year holiday. > > On 2021/1/25 22:51, Vincent Guittot wrote: > > On Mon, 25 Jan 2021 at 15:00, Li, Aubrey <aubrey.li@linux.intel.com> wrote: > >> > >> On 2021/1/25 18:56, Vincent Guittot wrote: > >>> On Mon, 25 Jan 2021 at 06:50, Aubrey Li <aubrey.li@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 > > > >> 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 > > Sure, I can take your approach, :) > > >>> > >>> --- a/kernel/sched/fair.c > >>> +++ b/kernel/sched/fair.c > >>> @@ -7688,6 +7688,16 @@ static int detach_tasks(struct lb_env *env) > >>> > >>> lockdep_assert_held(&env->src_rq->lock); > >>> > >>> + /* > >>> + * Another CPU has emptied this runqueue in the meantime. > >>> + * Just return and leave the load_balance properly. > >>> + */ > >>> + if (env->src_rq->nr_running <= 1 && !env->loop) { > > May I know why !env->loop is needed here? IIUC, if detach_tasks is invoked
IIRC, my point was to do the test only when trying to detach the 1st task. A lot of things can happen when a break is involved but TBH I can't remember a precise UC. It may be over cautious
> from LBF_NEED_BREAK, env->loop could be non-zero, but as long as src_rq's > nr_running <=1, we should return immediately with LBF_ALL_PINNED flag cleared. > > How about the following change? > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 04a3ce20da67..1761d33accaa 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7683,8 +7683,11 @@ static int detach_tasks(struct lb_env *env) > * We don't want to steal all, otherwise we may be treated likewise, > * which could at worst lead to a livelock crash. > */ > - if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1) > + if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1) {
IMO, we must do the test before: while (!list_empty(tasks)) {
because src_rq might have become empty if waiting tasks have been pulled by another cpu and the running one became idle in the meantime
> + /* Clear the flag as we will not test any task */ > + env->flag &= ~LBF_ALL_PINNED; > break; > + } > > p = list_last_entry(tasks, struct task_struct, se.group_node); > > Thanks, > -Aubrey
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