Re: [PATCH v4 01/12] sched: fix imbalance flag reset
From: Wanpeng Li
Date: Tue Nov 25 2014 - 05:14:08 EST
Hi Vincent,
On 11/25/14, 5:04 PM, Vincent Guittot wrote:
On 25 November 2014 at 00:47, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
Hi Vincent,
On 7/29/14, 1:51 AM, 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.
We need to reset of the imbalance flag as soon as we have reached a
balanced
state. If all tasks are pinned, we don't consider that as a balanced state
and
let the imbalance flag set.
Reviewed-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
kernel/sched/fair.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 923fe32..7eb9126 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6672,10 +6672,8 @@ static int load_balance(int this_cpu, struct rq
*this_rq,
if (sd_parent) {
int *group_imbalance =
&sd_parent->groups->sgc->imbalance;
- if ((env.flags & LBF_SOME_PINNED) && env.imbalance
0) {
+ if ((env.flags & LBF_SOME_PINNED) && env.imbalance
0)
*group_imbalance = 1;
- } else if (*group_imbalance)
- *group_imbalance = 0;
As you mentioned above " We need to reset of the imbalance flag as soon as
we have reached a balanced state. " I think the codes before your patch have
already do this, where I miss? Great thanks for your patient. ;-)
The previous code was called only when busiest->nr_running > 1. The
background activity will be on the rq only 1 tick per few seconds and
we will set qroup_imbalance when the background activity is on the rq.
Then, during the next load balances, the qroup_imbalance is still set
but we can't clear qroup_imbalance because we have only 1 task per rq
There is no load balance I think since busiest->nr_running > 1 is not
true even if the patch is not applied.
Regards,
Wanpeng Li
Regards,
Vincent
Regards,
Wanpeng Li
}
/* All tasks on this runqueue were pinned by CPU affinity
*/
@@ -6686,7 +6684,7 @@ static int load_balance(int this_cpu, struct rq
*this_rq,
env.loop_break = sched_nr_migrate_break;
goto redo;
}
- goto out_balanced;
+ goto out_all_pinned;
}
}
@@ -6760,6 +6758,23 @@ static int load_balance(int this_cpu, struct rq
*this_rq,
goto out;
out_balanced:
+ /*
+ * We reach balance although we may have faced some affinity
+ * constraints. Clear the imbalance flag if it was set.
+ */
+ if (sd_parent) {
+ int *group_imbalance = &sd_parent->groups->sgc->imbalance;
+
+ if (*group_imbalance)
+ *group_imbalance = 0;
+ }
+
+out_all_pinned:
+ /*
+ * We reach balance because all tasks are pinned at this level so
+ * we can't migrate them. Let the imbalance flag set so parent
level
+ * can try to migrate them.
+ */
schedstat_inc(sd, lb_balanced[idle]);
sd->nr_balance_failed = 0;
--
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