| Message ID | 5383065E.9030601@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 26 May 2014 11:16, Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote: > On 05/26/2014 01:19 PM, Vincent Guittot wrote: >> On 25 May 2014 12:33, Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote: >>> Hi Vincent, >>> >>> On 05/23/2014 09:22 PM, Vincent Guittot wrote: >>>> The imbalance flag can stay set whereas there is no imbalance. >>>> >>>> Let assume that we have 3 tasks that run on a dual cores /dual cluster system. >>>> We will have some idle load balance which are triggered during tick. >>>> Unfortunately, the tick is also used to queue background work so we can reach >>>> the situation where short work has been queued on a CPU which already runs a >>>> task. The load balance will detect this imbalance (2 tasks on 1 CPU and an idle >>>> CPU) and will try to pull the waiting task on the idle CPU. The waiting task is >>>> a worker thread that is pinned on a CPU so an imbalance due to pinned task is >>>> detected and the imbalance flag is set. >>>> Then, we will not be able to clear the flag because we have at most 1 task on >>>> each CPU but the imbalance flag will trig to useless active load balance >>>> between the idle CPU and the busy CPU. >>> >>> Why do we do active balancing today when there is at-most 1 task on the >>> busiest cpu? Shouldn't we be skipping load balancing altogether? If we >>> do active balancing when the number of tasks = 1, it will lead to a ping >>> pong right? >> >> That's the purpose of the patch to prevent this useless active load >> balance. When the imbalance flag is set, an active load balance is >> triggered whatever the load balance is because of pinned tasks that >> prevents a balance state. > > No I mean this: > > sched:Do not continue load balancing when the busiest cpu has one > running task But you can have situation where you have to migrate the task even if the busiest CPU has only 1 task. The use of imbalance flag is one example. You can also be in a situation where the busiest group has too much load compared to the local group and the busiest CPU even with 1 task (at this instant) has been selected to move task away from the busiest group Vincent > > From: Preeti U Murthy <preeti@linux.vnet.ibm.com> > > > --- > kernel/sched/fair.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index c9617b7..b175333 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6626,6 +6626,8 @@ more_balance: > } > goto out_balanced; > } > + } else { > + goto out; > } > > if (!ld_moved) { > > > } > > Regards > Preeti U Murthy >> >> Vincent >> >>> >>> Regards >>> Preeti U Murthy >>> >> >
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c9617b7..b175333 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6626,6 +6626,8 @@ more_balance: } goto out_balanced; } + } else { + goto out; } if (!ld_moved) {