Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only one task
- From: Gautham R Shenoy <ego@xxxxxxxxxx>
- Date: Fri, 12 Sep 2008 12:05:39 +0530
On Fri, Sep 05, 2008 at 07:23:44PM +0200, Peter Zijlstra wrote:
On Fri, 2008-09-05 at 17:13 +0200, Peter Zijlstra wrote:
On Fri, 2008-09-05 at 18:00 +0530, Gautham R Shenoy wrote:
sched: Fix __load_balance_iterator() for cfq with only one task.
From: Gautham R Shenoy <ego@xxxxxxxxxx>
The __load_balance_iterator() returns a NULL when there's only one
sched_entity which is a task. It is caused by the following code-path.
/* Skip over entities that are not tasks */
do {
se = list_entry(next, struct sched_entity, group_node);
next = next->next;
} while (next != &cfs_rq->tasks && !entity_is_task(se));
if (next == &cfs_rq->tasks)
return NULL;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This will return NULL even when se is a task.
As a side-effect, there was a regression in sched_mc behavior since 2.6.25,
since iter_move_one_task() when it calls load_balance_start_fair(),
would not get any tasks to move!
Fix this by checking if the last entity was a task or not.
Gregory did a similar fix a while ago, but that caused grief of some
kind..
Greg, can you recollect why we pulled it? I can't seem to find it.
Gregory pointed me to this thread:
http://lkml.org/lkml/2008/8/11/81
ego, can you run sysbench to confirm?
Am planning to run it today.
Mike, with what --oltp-* mode did you run the sysbench test?
That aside, if Mike's analysis is correct regarding the client/server
pairs not running on the same CPU as buddies, shouldn't this be fixed in a
higher level routine rather than have this anomaly in
__load_balancer_iterator(), which is supposed to return the runnable
tasks in the cfs_rq ?
It's current behavior is that __load_balancer_iterator() will
return NULL even if the last entity in the list is a runnable task.
This behavior clearly hinders sched_mc powersavings from migrating
a sole remaining task from a powersavings-sched_domain in-order
to evacuate that domain and put all the CPUs of the domain into a
low-power state.
Aside from that this patch looks fine..
Signed-off-by: Gautham R Shenoy <ego@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
---
kernel/sched_fair.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index fb8994c..f1c96e3 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1451,7 +1451,7 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
next = next->next;
} while (next != &cfs_rq->tasks && !entity_is_task(se));
- if (next == &cfs_rq->tasks)
+ if (next == &cfs_rq->tasks && !entity_is_task(se))
return NULL;
cfs_rq->balance_iterator = next;
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--
Thanks and Regards
gautham
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