Re: per-cpu blk_plug_list

[Andrew Morton <akpm@osdl.org>]

"Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
>
> I don't understand the proposal here.  There is a per-device lock
> already.  But the plugged queue need to be on some list outside itself
> so a group of them can be unplugged later on to flush all the I/O.


here's the proposal:


Regarding this:

 http://www.ussg.iu.edu/hypermail/linux/kernel/0403.0/0179.html

 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).

 We discussed this maybe a year ago with Jens but decided against it for
 complexity reasons, but gee, dm_table_any_congested() isn't complex.  Do we
 forsee any particular problem with this?