Messages in this thread | | | Date | Mon, 12 Aug 2013 16:43:09 +0200 | From | Peter Zijlstra <> | Subject | Re: false nr_running check in load balance? |
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On Tue, Aug 06, 2013 at 09:23:46PM +0800, Lei Wen wrote: > Hi Paul, > > I notice in load_balance function, it would check busiest->nr_running > to decide whether to perform the real task movement. > > But in some case, I saw the nr_running is not matching with > the task in the queue, which seems make scheduler to do many redundant > checking. > What I means is like there is only one task in the queue, but nr_running > shows it has two. So if that task cannot be moved, it would be still checked > for twice. > > With further checking, I find there is one patch you submit before: > commit 953bfcd10e6f3697233e8e5128c611d275da39c1 > Author: Paul Turner <pjt@google.com> > Date: Thu Jul 21 09:43:27 2011 -0700 > > sched: Implement hierarchical task accounting for SCHED_OTHER > > In this patch, you increase nr_running when enqueue enqueue_task_stop, > which is the reason nr_running is increase while task not be increased. > It is true at that time, the stopper has been waken up and enqueue again > into cpu, and do the migration job. So the logic should be right there. > > My question is whether we could change the judgment into cfs_rq->nr_running? > Since the load_balance is only for cfs, right? > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index bb456f4..ffc0d35 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5096,7 +5096,7 @@ redo: > schedstat_add(sd, lb_imbalance[idle], env.imbalance); > > ld_moved = 0; > - if (busiest->nr_running > 1) { > + if (busiest->cfs.nr_running > 1) { > /* > * Attempt to move tasks. If find_busiest_group has found > * an imbalance but busiest->nr_running <= 1, the group is >
Not quite right; I think you need busiest->cfs.h_nr_running. cfs.nr_running is the number of entries running in this 'group'. If you've got nested groups like:
'root' \ 'A' / \ t1 t2
root.nr_running := 1 'A', even though you've got multiple running tasks.
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