Re: [RFC 2/2] x86_64: expand kernel stack to 16K

[Dave Chinner]

On Fri, May 30, 2014 at 08:06:53PM -0600, Jens Axboe wrote:
> On 2014-05-28 20:42, Linus Torvalds wrote:
> >Well, we've definitely have had some issues with deeper callchains
> >with md, but I suspect virtio might be worse, and the new blk-mq code
> >is lilkely worse in this respect too.
> 
> I don't think blk-mq is worse than the older stack, in fact it
> should be better. The call chains are shorter, and a lot less cruft
> on the stack. Historically the stack issues have been nested
> devices, however. And for sync IO, we do run it inline, so if the
> driver chews up a lot of stack, well...

Hi Jens - as we found out with the mm code, there's a significant
disconnect between what the code looks like (i.e. it may use very
little stack directly) and what the compiler is generating.

Before blk-mq:

  9)     3952     112   scsi_request_fn+0x4b/0x490
 10)     3840      32   __blk_run_queue+0x37/0x50
 11)     3808      64   queue_unplugged+0x39/0xb0
 12)     3744     112   blk_flush_plug_list+0x20b/0x240

Now with blk-mq:

  3)     4672      96   virtio_queue_rq+0xd2/0x1e0
  4)     4576     128   __blk_mq_run_hw_queue+0x1f0/0x3e0
  5)     4448      16   blk_mq_run_hw_queue+0x35/0x40
  6)     4432      80   blk_mq_insert_requests+0xc7/0x130
  7)     4352      96   blk_mq_flush_plug_list+0x129/0x140
  8)     4256     112   blk_flush_plug_list+0xe7/0x230

So previously flushing a plug used rough 200 bytes of stack.  With
blk-mq, it's over 400 bytes. IOWs, blk-mq has more than doubled the
block layer stack usage...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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