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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:
    >
    > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.4-rc1/2.6.4-rc1-mm1/broken-out/queue-congestion-dm-implementation.patch
    >
    > 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
    https://www.redhat.com/archives/linux-lvm/2004-February/msg00203.html
    (though I'd replace "rw" with "bdi_bits" like in the current patch).

    Mike.
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