Re: [PATCH 3/5] workqueue: fixup existing pool->node

[Tejun Heo]

On Fri, Dec 12, 2014 at 06:19:53PM +0800, Lai Jiangshan wrote:
> Yasuaki Ishimatsu hit a bug when the numa mapping between CPU and node
> is changed.  And the previous path fixup wq_numa_possible_cpumask.
> (See more information form the changelog of that patch)
> 
> After wq_numa_possible_cpumask was updated, the new pool->node will be
> correct, but the existing pools (and workers) are still running, some of
> them are running with the wrong pool->node, or even worse, with pool->node
> which is quitted node, they create_worker() on wrong pool->node.
> These create_worker() may create workers on wrong node or failed without
> any progress (when with pool->node which is quitted node).
> 
> So we need to update the pool->node when the numa mapping is changed.
> 
> We simply re-calc the pool->node when the numa mapping changed. It reuses
> the code from get_unbound_pool() for unbound pool.

I don't get this patch.  If a node is gone, all its cpus would be gone
and the pool should be discarded.  If a new node comes online with
different mappings, new sets of pools should serve them instead of
recycling the old ones.  Wouldn't it make a lot more sense to make
sure we don't reuse the pools w/ old mappings for new pwqs?

Thanks.

-- 
tejun
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