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For example, the task list >>>> may have four tasks at the beginning, it may becomes to two >>>> during the scanning in detach_tasks(): >>>> Task list at beginning : "ABCD" >>>> Task list in scanning : "CD" >>>> >>>> (ABCD stands for differnt tasks.) >>>> >>>> In this scenario, the env->loop_max is still four, so >>>> detach_tasks() may scan twice for some tasks: >>>> the scanning order maybe : "DCDC" >>>> >>> Huh, a quick hacky test suggests this isn't /too/ hard to trigger; I get >>> about one occurrence every two default hackbench run (~200ms) on my dummy >>> QEMU setup. >>> >>> Have you seen this happen on your workloads or did you find this while >>> staring at the code? >> I found this issue in my Specjbb2015 test.  It is very easy to trigger. >> > That would be good to include in the changelog. okay. >> I noticed many times in the test. >> >> I even found that: >> >>         At the beginning: env->loop_max is 12. >> >>        When the detach_tasks() scans: the real task list shrink to 5. >> > That's set using rq->nr_running which includes more than just the CFS > tasks, and looking at the git history it looks like it's almost always been > the case. > > Looks like env->loop_max really is only used for detach_tasks(), so perhaps > a "better" fix would be something like the below, so that we can't iterate > more than length(env->src_rq->cfs_tasks). That is, assuming > > rq->cfs.h_nr_queued == length(rq->cfs_tasks) > > --- > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index b9b4bbbf0af6f..32ae24aa377ca 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -11687,7 +11687,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq, > * still unbalanced. ld_moved simply stays zero, so it is > * correctly treated as an imbalance. > */ > - env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running); > + env.loop_max = min(sysctl_sched_nr_migrate, busiest->cfs.h_nr_queued); I tested this patch, it did not work. I still can catch lots of occurrences of this issue in Specjbb test. IMHO, the root cause of this issue is env.loop_max is set out of the rq's lock. Even we set env.loop_max to busiest->cfs.h_nr_queued, the real tasks length still can shrink in other places. Btw, the real tasks's length can also bigger then env.loop_max.  For example, we set env.loop_max with 5, when detach_tasks() runs, the real tasks' length can be changed to 7. Thanks Huang Shijie > > more_balance: > rq_lock_irqsave(busiest, &rf); >