Date:   Wed, 5 Dec 2018 13:38:17 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     mhocko@...nel.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH] mm/alloc: fallback to first node if the wanted node offline

On Tue, Dec 4, 2018 at 4:56 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Tue 04-12-18 16:20:32, Pingfan Liu wrote:
> > On Tue, Dec 4, 2018 at 3:22 PM Michal Hocko <mhocko@...nel.org> wrote:
> > >
> > > On Tue 04-12-18 11:05:57, Pingfan Liu wrote:
> > > > During my test on some AMD machine, with kexec -l nr_cpus=x option, the
> > > > kernel failed to bootup, because some node's data struct can not be allocated,
> > > > e.g, on x86, initialized by init_cpu_to_node()->init_memory_less_node(). But
> > > > device->numa_node info is used as preferred_nid param for
> > > > __alloc_pages_nodemask(), which causes NULL reference
> > > >   ac->zonelist = node_zonelist(preferred_nid, gfp_mask);
> > > > This patch tries to fix the issue by falling back to the first online node,
> > > > when encountering such corner case.
> > >
> > > We have seen similar issues already and the bug was usually that the
> > > zonelists were not initialized yet or the node is completely bogus.
> > > Zonelists should be initialized by build_all_zonelists quite early so I
> > > am wondering whether the later is the case. What is the actual node
> > > number the device is associated with?
> > >
> > The device's node num is 2. And in my case, I used nr_cpus param. Due
> > to init_cpu_to_node() initialize all the possible node.  It is hard
> > for me to figure out without this param, how zonelists is accessed
> > before page allocator works.
>
> I believe we should focus on this. Why does the node have no zonelist
> even though all zonelists should be initialized already? Maybe this is
> nr_cpus pecularity and we do not initialize all the existing numa nodes.
> Or maybe the device is associated to a non-existing node with that
> setup. A full dmesg might help us here.
>
Requiring the machine again, and I got the following without nr_cpus option
[root@...l-per7425-03 ~]# cd /sys/devices/system/node/
[root@...l-per7425-03 node]# ls
has_cpu  has_memory  has_normal_memory  node0  node1  node2  node3
node4  node5  node6  node7  online  possible  power  uevent
[root@...l-per7425-03 node]# cat has_cpu
0-7
[root@...l-per7425-03 node]# cat has_memory
1,5
[root@...l-per7425-03 node]# cat online
0-7
[root@...l-per7425-03 node]# cat possible
0-7
And lscpu shows the following numa-cpu info:
NUMA node0 CPU(s):     0,8,16,24
NUMA node1 CPU(s):     2,10,18,26
NUMA node2 CPU(s):     4,12,20,28
NUMA node3 CPU(s):     6,14,22,30
NUMA node4 CPU(s):     1,9,17,25
NUMA node5 CPU(s):     3,11,19,27
NUMA node6 CPU(s):     5,13,21,29
NUMA node7 CPU(s):     7,15,23,31

For the full panic message (I masked some hostname info with xx),
please see the attachment.
In a short word, it seems a problem with nr_cpus, if without this
option, the kernel can bootup correctly.

> > > Your patch is not correct btw, because we want to fallback into the node in
> > > the distance order rather into the first online node.
> > > --
> > What about this:
> > +extern int find_next_best_node(int node, nodemask_t *used_node_mask);
> > +
> >  /*
> >   * We get the zone list from the current node and the gfp_mask.
> >   * This zone list contains a maximum of MAXNODES*MAX_NR_ZONES zones.
> > @@ -453,6 +455,11 @@ static inline int gfp_zonelist(gfp_t flags)
> >   */
> >  static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
> >  {
> > +       if (unlikely(!node_online(nid))) {
> > +               nodemask_t used_mask;
> > +               nodes_complement(used_mask, node_online_map);
> > +               nid = find_next_best_node(nid, &used_mask);
> > +       }
> >         return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);
> >  }
> >
> > I just finished the compiling, not test it yet, since the machine is
> > not on hand yet. It needs some time to get it again.
>
> This is clearly a no-go. nodemask_t can be giant and you cannot have it
> on the stack for allocation paths which might be called from a deep

What about the static variable
static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
 {
+       if (unlikely(!node_online(nid))) {
+                   WARN_ONCE(1, "Try to alloc mem from not online node\n");
+                   nid = find_best_node(nid);
+       }
        return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);
 }

+int find_best_node(int node)
+{
+       static nodemask_t used_mask;
+       static DEFINE_SPINLOCK(lock);
+       static int best_neigh[MAX_NUMNODES] = {0};
+       int nid;
+
+       spin_lock(&lock);
+       if (likely( best_neigh[node] != 0)) {
+               nid = best_neigh[node];
+               goto unlock;
+       }
+       nodes_complement(used_mask, node_online_map);
+       nid = find_next_best_node(node, &used_mask);
+       best_neigh[node] = nid;
+
+unlock:
+       spin_unlock(&lock);
+       return nid;
+}

> stack already. Also this is called from the allocator hot paths and each
> branch counts.
>
I am puzzling about this. Does unlikely work for it?

Thanks,
Pingfan

View attachment "nrcpus.txt" of type "text/plain" (65200 bytes)

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