Re: per-cpu blk_plug_list

[Miquel van Smoorenburg <miquels@cistron.nl>]

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.