Re: [PATCH] workqueue: update numa affinity when node hotplug
From: Kamezawa Hiroyuki
Date: Mon Mar 02 2015 - 03:52:53 EST
On 2015/02/27 20:54, Tejun Heo wrote:
On Fri, Feb 27, 2015 at 06:04:52PM +0800, Gu Zheng wrote:
Yasuaki Ishimatsu found that with node online/offline, cpu<->node
relationship is established. Because workqueue uses a info which was
established at boot time, but it may be changed by node hotpluging.
I've asked this a couple times now but can somebody please justify why
cpu<->node relationship needs to change? If it has to change, that's
okay but let's please first make sure that we understand why such
thing is necessary so that we can figure out what kind of facilities
are necessary for such dynamism.
Let me start from explaining current behavior.
- cpu-id is determined when a new processor(lapicid/x2apicid) is founded.
cpu-id<->nodeid relationship is _not_ recorded.
- node-id is determined when a new pxm(firmware info) is founded.
pxm<->nodeid relationship is recorded.
By this, there are 2 cases of cpu<->nodeid change.
Case A) In x86, cpus on memory-less nodes are all tied to existing nodes(round robin).
At memory-hotadd happens and a new node comes, cpus are moved to a newly added node
based on pxm.
Case B) Adding a node after removing another node, if pxm of them were different from
each other, cpu<->node relatiionship changes.
I personally thinks proper fix is building persistent cpu-id <-> lapicid relationship as
pxm does rather than creating band-aid.
But I have no good idea to hanlde case A), which is used now.
Hm, Case-A will not be problem for the case of workqueue's kmalloc because a node with
ZONE_NORMAL cannot be removed.
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