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

[Jens Axboe]

On 2011-04-12 15:40, Dave Chinner wrote:
> 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@xxxxxxxxxxxxx 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. :)

Do you have traces from 2.6.38, or are you just doing them now?

The path you quote above should not go into reclaim, it's a GFP_ATOMIC
allocation.

-- 
Jens Axboe

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