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Re: [libvirt] [PATCH] util: Correct the NUMA node range checking

On 23/01/14 08:04, Eric Blake wrote:
On 01/22/2014 04:45 AM, Osier Yang wrote:
There are 2 issues here: First we shouldn't add "1" to the return
value of numa_max_node(), since the semanteme of the error message

was changed, it's not saying about the number of total NUMA nodes
anymore.  Second, the value of "bit" is the position of the first
bit which exceeds either numa_max_node() or NUMA_NUM_NODES, it can
be any number in the range, so saying "bigger than $bit" is quite
confused now. For example, assuming there is a NUMA machine which
has 10 NUMA nodes, and one specifies the "nodeset" as "0,5,88",
the error message will be like:

Nodeset is out of range, host cannot support NUMA node bigger than 88

It sounds like all NUMA node number less than 88 is fine, but
actually the maximum NUMA node number the machine supports is 9.

This patch fixes the issues by removing the addition with "1" and
simplifies the error message as "NUMA node $bit is out of range".
  src/util/virnuma.c | 8 ++------
  1 file changed, 2 insertions(+), 6 deletions(-)
-    maxnode = numa_max_node() + 1;
      /* Convert nodemask to NUMA bitmask. */
      bit = -1;
      while ((bit = virBitmapNextSetBit(tmp_nodemask, bit)) >= 0) {
-        if (bit > maxnode || bit > NUMA_NUM_NODES) {
+        if (bit > numa_max_node() || bit > NUMA_NUM_NODES) {
This calls numa_max_node() in a loop, where we used to call it just
once.  Since this is third-party code, do we know how efficient it is?
It may be smarter to cache the results once than to call every iteration
of the loop, to avoid O(n*2) behavior on large hosts.

For that matter, can't we just set the max node to the smaller of
numa_max_node() and NUMA_NUM_NODES up front, and avoid the || in the loop?

Good idea, pushed with the fix, along with a bit change in the commit log.


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