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    SubjectRe: [PATCH 3/9] mm, page_alloc: do not set_cpu_numa_mem on empty nodes initialization
    On Fri, Jul 14, 2017 at 02:39:41PM +0200, Michal Hocko wrote:
    > On Fri 14-07-17 13:32:42, Mel Gorman wrote:
    > > On Fri, Jul 14, 2017 at 12:50:04PM +0200, Michal Hocko wrote:
    > > > On Fri 14-07-17 10:48:10, Mel Gorman wrote:
    > > > > On Fri, Jul 14, 2017 at 10:00:00AM +0200, Michal Hocko wrote:
    > > > > > From: Michal Hocko <mhocko@suse.com>
    > > > > >
    > > > > > __build_all_zonelists reinitializes each online cpu local node for
    > > > > > CONFIG_HAVE_MEMORYLESS_NODES. This makes sense because previously memory
    > > > > > less nodes could gain some memory during memory hotplug and so the local
    > > > > > node should be changed for CPUs close to such a node. It makes less
    > > > > > sense to do that unconditionally for a newly creaded NUMA node which is
    > > > > > still offline and without any memory.
    > > > > >
    > > > > > Let's also simplify the cpu loop and use for_each_online_cpu instead of
    > > > > > an explicit cpu_online check for all possible cpus.
    > > > > >
    > > > > > Signed-off-by: Michal Hocko <mhocko@suse.com>
    > > > > > ---
    > > > > > mm/page_alloc.c | 6 ++----
    > > > > > 1 file changed, 2 insertions(+), 4 deletions(-)
    > > > > >
    > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > > > > > index 7746824a425d..ebc3311555b1 100644
    > > > > > --- a/mm/page_alloc.c
    > > > > > +++ b/mm/page_alloc.c
    > > > > > @@ -5096,10 +5096,8 @@ static int __build_all_zonelists(void *data)
    > > > > >
    > > > > > build_zonelists(pgdat);
    > > > > > }
    > > > > > - }
    > > > > >
    > > > > > #ifdef CONFIG_HAVE_MEMORYLESS_NODES
    > > > > > - for_each_possible_cpu(cpu) {
    > > > > > /*
    > > > > > * We now know the "local memory node" for each node--
    > > > > > * i.e., the node of the first zone in the generic zonelist.
    > > > > > @@ -5108,10 +5106,10 @@ static int __build_all_zonelists(void *data)
    > > > > > * secondary cpus' numa_mem as they come on-line. During
    > > > > > * node/memory hotplug, we'll fixup all on-line cpus.
    > > > > > */
    > > > > > - if (cpu_online(cpu))
    > > > > > + for_each_online_cpu(cpu)
    > > > > > set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
    > > > > > - }
    > > > > > #endif
    > > > > > + }
    > > > > >
    > > > >
    > > > > This is not as clear a benefit. For each online node, we now go through
    > > > > all online CPUs once per node. There would be some rationale for using
    > > > > for_each_online_cpu.
    > > >
    > > > I am not sure I understand. I am using for_each_online_cpu...
    > >
    > > Yes, but within a loop that looks like
    > >
    > > for_each_online_node(nid)
    > > ...
    > > for_each_online_cpu(cpu)
    > >
    > > Or maybe you aren't because we are looking at different baselines. I had
    > > minor fuzz and conflicts applying the series.
    >
    > The current mmotm after this patch looks like this
    > if (self && !node_online(self->node_id)) {
    > build_zonelists(self);
    > } else {
    > for_each_online_node(nid) {
    > pg_data_t *pgdat = NODE_DATA(nid);
    >
    > build_zonelists(pgdat);
    > }
    >
    > #ifdef CONFIG_HAVE_MEMORYLESS_NODES
    > /*
    > * We now know the "local memory node" for each node--
    > * i.e., the node of the first zone in the generic zonelist.
    > * Set up numa_mem percpu variable for on-line cpus. During
    > * boot, only the boot cpu should be on-line; we'll init the
    > * secondary cpus' numa_mem as they come on-line. During
    > * node/memory hotplug, we'll fixup all on-line cpus.
    > */
    > for_each_online_cpu(cpu)
    > set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
    > #endif
    > }
    >

    Ok, that makes more sense.

    Acked-by: Mel Gorman <mgorman@suse.de>

    --
    Mel Gorman
    SUSE Labs

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