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

[Jens Axboe]

On 2011-04-12 15:31, Dave Chinner wrote:
> On Tue, Apr 12, 2011 at 02:58:46PM +0200, Jens Axboe wrote:
>> On 2011-04-12 14:41, 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:
>>>>>>>> Great, once you do that and XFS kills the blk_flush_plug() calls too,
>>>>>>>> then we can remove that export and make it internal only.
>>>>>>>
>>>>>>> Linus pulled the tree, so they are gone now.  Btw, there's still some
>>>>>>> bits in the area that confuse me:
>>>>>>
>>>>>> Great!
>>>>>>
>>>>>>>  - what's the point of the queue_sync_plugs?  It has a lot of comment
>>>>>>>    that seem to pre-data the onstack plugging, but except for that
>>>>>>>    it's trivial wrapper around blk_flush_plug, with an argument
>>>>>>>    that is not used.
>>>>>>
>>>>>> There's really no point to it anymore. It's existance was due to the
>>>>>> older revision that had to track write requests for serializaing around
>>>>>> a barrier. I'll kill it, since we don't do that anymore.
>>>>>>
>>>>>>>  - is there a good reason for the existance of __blk_flush_plug?  You'd
>>>>>>>    get one additional instruction in the inlined version of
>>>>>>>    blk_flush_plug when opencoding, but avoid the need for chained
>>>>>>>    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.)
>>>>>
>>>>> I've seen traces where mempool_alloc_slab enters direct reclaim
>>>>> which adds another 1.5k of stack usage to this path. So I'm
>>>>> extremely concerned that you've just reduced the stack available to
>>>>> every thread by at least 2.5k of space...
>>>>
>>>> Yeah, that does not look great. If this turns out to be problematic, we
>>>> can turn the queue runs from the unlikely case into out-of-line from
>>>> kblockd.
>>>>
>>>> But this really isn't that new, you could enter the IO dispatch path
>>>> when doing IO already (when submitting it). So we better be able to
>>>> handle that.
>>>
>>> The problem I see is that IO is submitted when there's plenty of
>>> stack available whould have previously been fine. However now it
>>> hits the plug, and then later on after the thread consumes a lot
>>> more stack it, say, waits for a completion. We then schedule, it
>>> unplugs the queue and we add the IO stack to a place where there
>>> isn't much space available.
>>>
>>> So effectively we are moving the places where stack is consumed
>>> about, and it's complete unpredictable where that stack is going to
>>> land now.
>>
>> Isn't that example fairly contrived?
> 
> I don't think so. e.g. in the XFS allocation path we do btree block
> readahead, then go do the real work. The real work can end up with a
> deeper stack before blocking on locks or completions unrelated to
> the readahead, leading to schedule() being called and an unplug
> being issued at that point.  You might think it contrived, but if
> you can't provide a guarantee that it can't happen then I have to
> assume it will happen.

If you ended up in lock_page() somewhere along the way, the path would
have been pretty much the same as it is now:

lock_page()
        __lock_page()
                __wait_on_bit_lock()
                        sync_page()
                                aops->sync_page();
                                        block_sync_page()
                                                __blk_run_backing_dev()

and the dispatch follows after that. If your schedules are only due to,
say, blocking on a mutex, then yes it'll be different. But is that
really the case?

I bet that worst case stack usage is exactly the same as before, and
that's the only metric we really care about.

> My concern is that we're already under stack space stress in the
> writeback path, so anything that has the potential to increase it
> significantly is a major worry from my point of view...

I agree on writeback being a worry, and that's why I made the change
(since it makes sense for other reasons, too). I just don't think we are
worse of than before.

>> If we ended up doing the IO
>> dispatch before, then the only difference now is the stack usage of
>> schedule() itself. Apart from that, as far as I can tell, there should
>> not be much difference.
> 
> There's a difference between IO submission and IO dispatch. IO
> submission is submit_bio thru to the plug; IO dispatch is from the
> plug down to the disk. If they happen at the same place, there's no
> problem. If IO dispatch is moved to schedule() via a plug....

The IO submission can easily and non-deterministically turn into an IO
dispatch, so there's no real difference for the submitter. That was the
case before. With the explicit plug now, you _know_ that the IO
submission is only that and doesn't include IO dispatch. Not until you
schedule() or call blk_finish_plug(), both of which are events that you
can control.

>>>> If it's a problem from the schedule()/io_schedule() path, then
>>>> lets ensure that those are truly unlikely events so we can punt
>>>> them to kblockd.
>>>
>>> Rather than wait for an explosion to be reported before doing this,
>>> why not just punt unplugs to kblockd unconditionally?
>>
>> Supposedly it's faster to do it inline rather than punt the dispatch.
>> But that may actually not be true, if you have multiple plugs going (and
>> thus multiple contenders for the queue lock on dispatch). So lets play
>> it safe and punt to kblockd, we can always revisit this later.
> 
> It's always best to play it safe when it comes to other peoples
> data....

Certainly, but so far I see no real evidence that this is in fact any
safer.

-- 
Jens Axboe

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