Re: [PATCH v2 3/7] x86, gfp: Cache best near node for memory allocation.

[Tang Chen]

Hi, tj

On 09/11/2015 03:29 AM, Tejun Heo wrote:
> Hello,
>
> On Thu, Sep 10, 2015 at 12:27:45PM +0800, Tang Chen wrote:
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index ad35f30..1a1324f 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -307,13 +307,19 @@ static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
> >   	if (nid < 0)
> >   		nid = numa_node_id();
> >   
> > +	if (!node_online(nid))
> > +		nid = get_near_online_node(nid);
> > +
> >   	return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
> >   }
> Why not just update node_data[]->node_zonelist in the first place?

zonelist will be rebuilt in __offline_pages() when the zone is not 
populated any more.

Here, getting the best near online node is for those cpus on memory-less 
nodes.

In the original code, if nid is NUMA_NO_NODE, the node the current cpu 
resides in
will be chosen. And if the node is memory-less node, the cpu will be 
mapped to its
best near online node.

But this patch-set will map the cpu to its original node, so 
numa_node_id() may return
a memory-less node to allocator. And then memory allocation may fail.

> Also, what's the synchronization rule here?  How are allocators
> synchronized against node hot [un]plugs?

The rule is: node_to_near_node_map[] array will be updated each time 
node [un]hotplug happens.

Now it is not protected by a lock. But I think acquiring a lock may 
cause performance regression
to memory allocator.

When rebuilding zonelist, stop_machine is used. So I think maybe 
updating the
node_to_near_node_map[] array at the same time when zonelist is rebuilt 
could be a good idea.

Thanks.



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