Re: [PATCH 05/10] block: remove per-queue plugging

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 12, 2011 at 02:28:31PM +0200, Jens Axboe wrote:
> On 2011-04-12 14:22, Dave Chinner wrote:
> > On Tue, Apr 12, 2011 at 10:36:30AM +0200, Jens Axboe wrote:
> >> On 2011-04-12 03:12, hch@infradead.org wrote:
> >>> On Mon, Apr 11, 2011 at 02:48:45PM +0200, Jens Axboe wrote:
> >>>    function calls.
> >>>  - Why is having a plug in blk_flush_plug marked unlikely?  Note that
> >>>    unlikely is the static branch prediction hint to mark the case
> >>>    extremly unlikely and is even used for hot/cold partitioning.  But
> >>>    when we call it we usually check beforehand if we actually have
> >>>    plugs, so it's actually likely to happen.
> >>
> >> The existance and out-of-line is for the scheduler() hook. It should be
> >> an unlikely event to schedule with a plug held, normally the plug should
> >> have been explicitly unplugged before that happens.
> > 
> > Though if it does, haven't you just added a significant amount of
> > depth to the worst case stack usage? I'm seeing this sort of thing
> > from io_schedule():
> > 
> >         Depth    Size   Location    (40 entries)
> >         -----    ----   --------
> >   0)     4256      16   mempool_alloc_slab+0x15/0x20
> >   1)     4240     144   mempool_alloc+0x63/0x160
> >   2)     4096      16   scsi_sg_alloc+0x4c/0x60
> >   3)     4080     112   __sg_alloc_table+0x66/0x140
> >   4)     3968      32   scsi_init_sgtable+0x33/0x90
> >   5)     3936      48   scsi_init_io+0x31/0xc0
> >   6)     3888      32   scsi_setup_fs_cmnd+0x79/0xe0
> >   7)     3856     112   sd_prep_fn+0x150/0xa90
> >   8)     3744      48   blk_peek_request+0x6a/0x1f0
> >   9)     3696      96   scsi_request_fn+0x60/0x510
> >  10)     3600      32   __blk_run_queue+0x57/0x100
> >  11)     3568      80   flush_plug_list+0x133/0x1d0
> >  12)     3488      32   __blk_flush_plug+0x24/0x50
> >  13)     3456      32   io_schedule+0x79/0x80
> > 
> > (This is from a page fault on ext3 that is doing page cache
> > readahead and blocking on a locked buffer.)

FYI, the next step in the allocation chain adds >900 bytes to that
stack:

$ cat /sys/kernel/debug/tracing/stack_trace
        Depth    Size   Location    (47 entries)
        -----    ----   --------
  0)     5176      40   zone_statistics+0xad/0xc0
  1)     5136     288   get_page_from_freelist+0x2cf/0x840
  2)     4848     304   __alloc_pages_nodemask+0x121/0x930
  3)     4544      48   kmem_getpages+0x62/0x160
  4)     4496      96   cache_grow+0x308/0x330
  5)     4400      80   cache_alloc_refill+0x21c/0x260
  6)     4320      64   kmem_cache_alloc+0x1b7/0x1e0
  7)     4256      16   mempool_alloc_slab+0x15/0x20
  8)     4240     144   mempool_alloc+0x63/0x160
  9)     4096      16   scsi_sg_alloc+0x4c/0x60
 10)     4080     112   __sg_alloc_table+0x66/0x140
 11)     3968      32   scsi_init_sgtable+0x33/0x90
 12)     3936      48   scsi_init_io+0x31/0xc0
 13)     3888      32   scsi_setup_fs_cmnd+0x79/0xe0
 14)     3856     112   sd_prep_fn+0x150/0xa90
 15)     3744      48   blk_peek_request+0x6a/0x1f0
 16)     3696      96   scsi_request_fn+0x60/0x510
 17)     3600      32   __blk_run_queue+0x57/0x100
 18)     3568      80   flush_plug_list+0x133/0x1d0
 19)     3488      32   __blk_flush_plug+0x24/0x50
 20)     3456      32   io_schedule+0x79/0x80

That's close to 1800 bytes now, and that's not entering the reclaim
path. If i get one deeper than that, I'll be sure to post it. :)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com