Messages in this thread | | | Date | Tue, 15 Apr 2014 10:35:29 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH 3/3] sched,numa: do not set preferred_node on migration to a second choice node |
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On 04/14/2014 08:56 AM, Peter Zijlstra wrote: > On Fri, Apr 11, 2014 at 01:00:29PM -0400, riel@redhat.com wrote: >> From: Rik van Riel <riel@redhat.com> >> >> 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 <riel@redhat.com> >> Tested-by: Vinod Chegu <chegu_vinod@hp.com> >> --- >> 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.
s/into/to/ makes the comment and the code match again :)
>> + * 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..
I have looked at the comment and the changelog some more, and it is not clear to me what you are missing, or what I could be explaining better...
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