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Re: [Xen-devel] [PATCH 3/3] page-alloc: Clamp get_free_buddy() to online nodes
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 25 Jun 2019 16:55:44 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, WeiLiu <wl@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 25 Jun 2019 15:55:57 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 25/06/2019 16:51, Jan Beulich wrote:
>>>> On 25.06.19 at 16:43, <andrew.cooper3@xxxxxxxxxx> wrote:
>> d->node_affinity defaults to NODE_MASK_ALL which has bits set outside of
>> node_online_map. This in turn causes the loop in get_free_buddy() to waste
>> effort iterating over offline nodes.
>>
>> Always clamp d->node_affinity to node_online_map when in use.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> despite ...
>
>> --- a/xen/common/page_alloc.c
>> +++ b/xen/common/page_alloc.c
>> @@ -811,11 +811,15 @@ static struct page_info *get_free_buddy(unsigned int
>> zone_lo,
>> const struct domain *d)
>> {
>> nodeid_t first, node = MEMF_get_node(memflags), req_node = node;
>> - nodemask_t nodemask = d ? d->node_affinity : node_online_map;
>> + nodemask_t nodemask;
>> unsigned int j, zone, nodemask_retry = 0;
>> struct page_info *pg;
>> bool use_unscrubbed = (memflags & MEMF_no_scrub);
>>
>> + /* Clamp nodemask to node_online_map and optionally d->node_affinity. */
>> + nodes_and(&nodemask, &node_online_map,
>> + d ? &d->node_affinity : &node_online_map);
> ... finding it a little odd (inefficient) to AND node_online_map with itself.
Well - there is an O(n) loop for the copy, and an O(n) loop for and, or
a single O(n) loop which does both when realising that foo = a & a is a
copy operation in disguise.
~Andrew
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