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SubjectRe: per-cpu blk_plug_list
According to Andrew Morton:
> And also having looked at Miquel's (currently slightly defective)
> implementation of the any_congested() API for devicemapper:
> I am thinking that an appropriate way of solving the blk_run_queues() lock
> contention problem is to nuke the global plug list altogther and make the
> unplug function a method in struct backing_device_info.
> This is conceptually the appropriate place to put it - it is almost always
> the case that when we run blk_run_queues() it is on behalf of an
> address_space, and the few remaining case can be simply deleted -
> mm/mempool.c is the last one I think.
> The implementation of backing_dev_info.unplug() would have to run the
> unplug_fn of every queue which contributes to the top-level queue (the
> thing which the address_space is sitting on top of).

But then you need a pointer to the queue. In that case,
you might as well put the congested_fn pointer in the request_queue
too. Then you get something like
(though I'd replace "rw" with "bdi_bits" like in the current patch).

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