Re: [PATCH]: New implementation of scsi_execute_async()

[Boaz Harrosh]

On 07/20/2009 08:54 PM, Vladislav Bolkhovitin wrote:
> Boaz Harrosh, on 07/19/2009 03:34 PM wrote:
>> On 07/16/2009 09:13 PM, Vladislav Bolkhovitin wrote:
>>> Boaz Harrosh, on 07/15/2009 12:19 PM wrote:
>>>>>>>  block/blk-map.c            |  536 +++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  drivers/scsi/scsi_lib.c    |  108 ++++++++-
>>>>>>>  include/linux/blkdev.h     |    6 
>>>>>>>  include/scsi/scsi_device.h |   11 
>>>>>>>  4 files changed, 660 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>> The scsi bits are not needed and just add complexity. 
>>>>>> allocate the request, call blk_rq_map_kern_sg() and
>>>>>> blk_execute_xxx directly from your driver. Since you exacly
>>>>>> know your code path lots of if(s) and flags are saved and that ugly
>>>>>> scsi_io_context allocation.
>>>>> Are you against any helper functions? scsi_lib.c has only scsi_execute_async(),
>>>>> which is a small helper function, which only hides internal machinery details.
>>>>> All the flags it has are needed and can't be avoided, especially
>>>>> SCSI_ASYNC_EXEC_FLAG_HAS_TAIL_SPACE_FOR_PADDING
>>>> Yes I'm sure. This will not be accepted. Just do it directly in the driver
>>>> like all other ULD's and block layer users. Use direct blk API. All these little
>>>> things and flags you are talking about are block specific, there is no single thing
>>>> SCSI about them.
>>> Hmm, I don't understand. What's wrong in this small helper function? Are 
>>> you against any helper functions, so, all users of, e.g., for_each_sg() 
>>> must instead open code the corresponding functionality?
>>>
>>> Do you want me to move scsi_execute_async() from scsi_lib.c to 
>>> scst_lib.c and rename it to scst_execute_async()? This is the maximum I 
>>> can do, because I need this functionality. But it will make all other 
>>> possible users to duplicate it.
> 
> OK, I'll do. But people who will miss it, will blame you by pretty bad
> words!
> 

Bring them on. I'm ready

>> Yes that's what I want, and if you can not save some of it's complexity
>> by, for example, merging of scsi_io_context with the request structure you
>> already have at scst level, or save the double-chained-callbacks at the end,
>> then you've done something wrong.
> 
> Are you sure??
> 
> As I already wrote, in SCST sense allocated on-demand only. It is done
> for performance reasons, because the sense is needed _very_ rarely and
> the buffer for it is quite big, so it's better to save those allocating,
> zeroing and CPU cache trashing. The fact that Linux SCSI ML requires
> sense buffer preallocation is for me something rather to be fixed.
> Pass-through SCST backend is used relatively rarely as well, so it's
> better to keep the sense preallocation for it in the separate code path.
> Then, if there is the sense preallocation, it doesn't matter to alloc
> with it few bytes more.
> 
> Is it getting clearer now?
> 

Yes it is now clear to me that scst is a mess. you cannot have more then
can_queue commands in-filght. 256 allocated mem_cache buffers of size 64-bytes
or size 96+64 bytes does not matter performance. It's the number of allocations
that count not the size of allocation, usually. If you want you can have a bigger
mem_cache in the scsi-pass-through case to hold a special-with-sense scst-command-
structure.

All drivers that care about sense find a way.

BTW you can call blk_execute_nowait without a sense buffer at all, it is optional.

>>> Does flag SCSI_ASYNC_EXEC_FLAG_AT_HEAD have nothing to do to SCSI, really?
>>>
>> Exactly, it's just a parameter at the call to blk_execute_nowait() with it's
>> own defines already. Nothing SCSI.
> 
> Well, going this way we can go too far.. Like, to assertion that SCSI
> HEAD OF QUEUE task attribute is a property of the Linux block layer as well.
> 

No "SCSI HEAD OF QUEUE task attribute" is a scsi attribute, none-scsi block
devices will not understand what it is. It is executed by the scsi-device.
Your flag will work for all block devices exactly because it is a parameter
to the block layer.

If you are implementing the scsi protocol to the letter then you must make sure
all subsystems comply for example pass the proper flag to the Linux block subsystem.

Hell look at your own code. And answer a simple question what does
SCSI_ASYNC_EXEC_FLAG_AT_HEAD do, and how does it do it:

+	blk_execute_rq_nowait(req->q, NULL, req,
+		flags & SCSI_ASYNC_EXEC_FLAG_AT_HEAD, scsi_end_async);

I'd say it's just a bit carrier to a boolean parameter at the call to
blk_execute_rq_nowait

>>>>> The allocation of scsi_io_context is unavoidable, because sense buffer should
>>>>> be allocated anyway.
>>>>>  
>>>> NO!, driver needs to do it's job and provide a sense buffer and call blk API
>>>> just like all the other drivers. this has nothing to do with SCSI.
>>> What job should a driver do? As I wrote, for SCST allocation of the 
>>> sense buffer is necessary in any case (in SCST sense is allocated on 
>>> demand only), so it doesn't matter to allocate few bytes more, if it 
>>> allows to hide unneeded low level details. The same, I guess, should be 
>>> true for all other possible users of this functionality.
>>>
>> Easiest, as all users do is: If you already have your scst request or
>> command, or what ever you call it. So add a sense_buffer array embedded
>> there, end of story no allocation needed.
>>
>> How can you be a scsi-target and return the proper errors upstream,
>> without digging into sense-buffer and error returns to produce the
>> correct report.
> 
> Most SCST backends don't deal with senses at all and generate them only
> at the latest stage of the commands processing to deliver errors (e.g.,
> file read errors) to initiators.
> 

Sense at the block-layer is optional just don't use it if you don't need
it.

>>>>>>> +/**
>>>>>>> + * blk_rq_unmap_kern_sg - "unmaps" data buffers in the request
>>>>>>> + * @req:	request to unmap
>>>>>>> + * @do_copy:	sets copy data between buffers, if needed, or not
>>>>>>> + *
>>>>>>> + * Description:
>>>>>>> + *    It frees all additional buffers allocated for SG->BIO mapping.
>>>>>>> + */
>>>>>>> +void blk_rq_unmap_kern_sg(struct request *req, int do_copy)
>>>>>>> +{
>>>>>>> +	struct scatterlist *hdr = (struct scatterlist *)req->end_io_data;
>>>>>>> +
>>>>>> You can't use req->end_io_data  here! req->end_io_data is reserved for
>>>>>> blk_execute_async callers. It can not be used for private block use.
>>>>> Why? I see blk_execute_rq() happily uses it. Plus, I implemented stacking of
>>>>> it in scsi_execute_async().
>>>>>
>>>> As I said users of blk_execute_rq_nowait() blk_execute_rq is a user of
>>>> blk_execute_rq_nowait just as the other guy.
>>>>
>>>> "implemented stacking" is exactly the disaster I'm talking about.
>>>> Also it totaly breaks the blk API. Now I need to to specific code
>>>> when mapping with this API as opposed to other mappings when I execute
>>>>
>>>>> Do you have better suggestion?
>>>> I have not look at it deeply but you'll need another system. Perhaps
>>>> like map_user/unmap_user where you give unmap_user the original bio.
>>>> Each user of map_user needs to keep a pointer to the original bio
>>>> on mapping. Maybe some other options as well. You can use the bio's
>>>> private data pointer, when building the first bio from scatter-list.
>>> I can't see how *well documented* stacking of end_io_data can be/lead to 
>>> any problem. All the possible alternatives I can see are worse:
>>>
>>> 1. Add in struct request one more field, like "void *blk_end_io_data" 
>>> and use it.
>>>
>>> 2. Duplicate code of bio's allocation and chaining 
>>> (__blk_rq_map_kern_sg()) for the copy case with additional code for 
>>> allocation and copying of the copy buffers on per-bio basis and use 
>>> bio->bi_private to track the copy info. Tejun Heo used this approach, 
>>> but he had only one bio without any chaining. With the chaining this 
>>> approach becomes terribly overcomplicated and ugly with *NO* real gain.
>>>
>>> Do you like any of them? If not, I'd like to see _practical_ suggestions.
>>>
>> Again all this is not needed and should be dropped it is already done
>> by bio bouncing. All you need to do is add the pages to bios, chain when
>> full, and call blk_make_request.
> 
> Can you show me a place where the bio bouncing, i.e.
> blk_queue_bounce(), does bouncing of misaligned buffers?
> 

It's there look harder

> I don't see such places. Instead, I see that all users of the block API,
> who cares about the alignment (sg, st, sr, etc.), directly or indirectly
> take care about it, by, e.g., switching to copying functions, before
> calling blk_queue_bounce(). See blk_rq_map_user_iov(), for instance.
> This is exactly what I implemented: handling of misaligned buffers on
> the layer upper blk_queue_bounce().
> 

Read the code again, I agree it is a grate mess but ...

For example what about filesystems they just put buffers on a bio and call
generic_make_request. sg st sr all do grate stupid things because of historical
reasons.

See the old scsi_execute_async of scsi_lib.c where was that alignment and
padding? there was not! did it work?

Look at blk_map_kern, where is alignment and padding handling? does it work?

> Thanks,
> Vlad
> 
> 

Boaz
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