On Fri, Apr 11, 2014 at 01:00:29PM -0400, [email protected] wrote: > From: Rik van Riel <[email protected]> > > Setting the numa_preferred_node for a task in task_numa_migrate > does nothing on a 2-node system. Either we migrate to the node > that already was our preferred node, or we stay where we were. > > On a 4-node system, it can slightly decrease overhead, by not > calling the NUMA code as much. Since every node tends to be > directly connected to every other node, running on the wrong > node for a while does not do much damage. > > However, on an 8 node system, there are far more bad nodes > than there are good ones, and pretending that a second choice > is actually the preferred node can greatly delay, or even > prevent, a workload from converging. > > The only time we can safely pretend that a second choice > node is the preferred node is when the task is part of a > workload that spans multiple NUMA nodes. > > Signed-off-by: Rik van Riel <[email protected]> > Tested-by: Vinod Chegu <[email protected]> > --- > kernel/sched/fair.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index babd316..302facf 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1301,7 +1301,16 @@ static int task_numa_migrate(struct task_struct *p) > if (env.best_cpu == -1) > return -EAGAIN; > > - sched_setnuma(p, env.dst_nid); > + /* > + * If the task is part of a workload that spans multiple NUMA nodes, > + * and is migrating into one of the workload's active nodes, remember
I read 'into' as: !node_isset(env.src_nid, ...) && node_isset(env.dst_nid, ...) The code doesn't seem to do this. > + * this node as the task's preferred numa node, so the workload can > + * settle down. > + * A task that migrated to a second choice node will be better off > + * trying for a better one later. Do not set the preferred node here. > + */ > + if (p->numa_group && node_isset(env.dst_nid, > p->numa_group->active_nodes)) > + sched_setnuma(p, env.dst_nid); OK, so I was totally confused on this one. What I missed was that we set the primary choice over in task_numa_placement(). I'm not really happy with the changelog; but I'm also struggling to identify what exactly is missing. Or rather, the thing makes me confused, and not feel like it actually explains it proper. That said; I tend to more or less agree with the actual change, but.. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
