On Fri, Jun 28, 2013 at 11:38:29AM +0530, Srikar Dronamraju wrote:
> * Mel Gorman <mgorman@xxxxxxx> [2013-06-26 15:38:01]:
>
> > This patch tracks what nodes numa hinting faults were incurred on. Greater
> > weight is given if the pages were to be migrated on the understanding
> > that such faults cost significantly more. If a task has paid the cost to
> > migrating data to that node then in the future it would be preferred if the
> > task did not migrate the data again unnecessarily. This information is later
> > used to schedule a task on the node incurring the most NUMA hinting faults.
> >
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > ---
> > include/linux/sched.h | 2 ++
> > kernel/sched/core.c | 3 +++
> > kernel/sched/fair.c | 12 +++++++++++-
> > kernel/sched/sched.h | 12 ++++++++++++
> > 4 files changed, 28 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index e692a02..72861b4 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1505,6 +1505,8 @@ struct task_struct {
> > unsigned int numa_scan_period;
> > u64 node_stamp; /* migration stamp */
> > struct callback_head numa_work;
> > +
> > + unsigned long *numa_faults;
> > #endif /* CONFIG_NUMA_BALANCING */
> >
> > struct rcu_head rcu;
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 67d0465..f332ec0 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1594,6 +1594,7 @@ static void __sched_fork(struct task_struct *p)
> > p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
> > p->numa_scan_period = sysctl_numa_balancing_scan_delay;
> > p->numa_work.next = &p->numa_work;
> > + p->numa_faults = NULL;
> > #endif /* CONFIG_NUMA_BALANCING */
> > }
> >
> > @@ -1853,6 +1854,8 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
> > if (mm)
> > mmdrop(mm);
> > if (unlikely(prev_state == TASK_DEAD)) {
> > + task_numa_free(prev);
> > +
> > /*
> > * Remove function-return probe instances associated with this
> > * task and put them back on the free list.
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7a33e59..904fd6f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -815,7 +815,14 @@ void task_numa_fault(int node, int pages, bool migrated)
> > if (!sched_feat_numa(NUMA))
> > return;
> >
> > - /* FIXME: Allocate task-specific structure for placement policy here */
> > + /* Allocate buffer to track faults on a per-node basis */
> > + if (unlikely(!p->numa_faults)) {
> > + int size = sizeof(*p->numa_faults) * nr_node_ids;
> > +
> > + p->numa_faults = kzalloc(size, GFP_KERNEL);
> > + if (!p->numa_faults)
> > + return;
> > + }
> >
> > /*
> > * If pages are properly placed (did not migrate) then scan slower.
> > @@ -826,6 +833,9 @@ void task_numa_fault(int node, int pages, bool migrated)
> > p->numa_scan_period + jiffies_to_msecs(10));
> >
> > task_numa_placement(p);
> > +
> > + /* Record the fault, double the weight if pages were migrated */
> > + p->numa_faults[node] += pages << migrated;
>
>
> Why are we doing this after the placement.
> I mean we should probably be doing this in the task_numa_placement,
>
Peter covered this.
> Since doubling the pages can have an effect on the preferred node. If we
> do it here, wont it end up in a case where the numa_faults on one node
> is actually higher but it may end up being not the preferred node?
>
Possibly but it's important to take into account the cost of migration. I
want to prefer keeping tasks on nodes that data was migrated to.
There is a much more serious problem with fault sampling that I have yet
to think of a good solution for. Consider a task that exhibits very high
locality and occasionally updates shared statistics. This hypothetical
workload is dominated by addressing a small array with the shared statistics
in a large array. In this case the PTE scanner will incur a larger number
of faults in the shared array even though it's less important to the
workload. The preferred node will be wrong in this case and is a much more
serious problem.
--
Mel Gorman
SUSE Labs
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