Messages in this thread | | | Subject | Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only one task | From | Peter Zijlstra <> | Date | Fri, 12 Sep 2008 08:56:15 +0200 |
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On Fri, 2008-09-12 at 12:05 +0530, Gautham R Shenoy wrote: > 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@in.ibm.com> > > > > > > > > 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.
Sure - there is buddy_hot in task_hot() to avoid moving buddies, and I think we should do something like this:
@@ -590,7 +602,7 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se) add_cfs_task_weight(cfs_rq, se->load.weight); cfs_rq->nr_running++; se->on_rq = 1; - list_add(&se->group_node, &cfs_rq->tasks); + list_add_tail(&se->group_node, &cfs_rq->tasks); }
static void
(most likely whitespace damaged)
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