Re: [PATCH 3/9] mm, page_alloc: do not set_cpu_numa_mem on empty nodes initialization

[Michal Hocko]

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@xxxxxxxx>
> > > > 
> > > > __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@xxxxxxxx>
> > > > ---
> > > >  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
	}

So for_each_online_cpu is called outside of the for_each_online_node.
Have a look at
git://git.kernel.org/pub/scm/linux/kernel/git/mhocko/mm.git
branch attempts/zonlists-build-simplification
-- 
Michal Hocko
SUSE Labs