[Lustre-devel] The good usage of lustre *_thread_info structure

[Nikita Danilov]

On Mar 26, 2011, at 17:22 , Aurélien Degrémont wrote:

> Le 26/03/2011 01:05, wangdi said:
>> This thread_info will be initialized in the beginning of the request
>> handling (ptlrpc_server_handle_request ->  lu_context_init, and attached
>> to the request), and currently the request will only be processed by a
>> single thread, i.e. no other threads will try to access the request and
>> the thread info. so it is safe to use this  info within the service thread.
>> 
>> This thread_info is actually designed for providing large temporary
>> memory to functions, so they can get these memories in a cheap way,
>> instead of allocating/freeing every time
> To be sure, what is the bad aspect we want to avoid here?

Kernel stack space is severely limited (4KB or 8KB) and dynamic memory allocation is too expensive for some  paths.

> 
> 
> Le 26/03/2011 08:52, Nikita Danilov said:
>> For the call chains within a layer it is up to the programmer to ascertain that the same element of lu_env::le_ctx is not re-used improperly. To some extent this can be done automatically, e.g., by introducing an interface to access sub-structures in struct mdt_thread_info, which would maintain a "busy" flag and check it on access.
> So it is rather a DIY :)
> 
> If I take an example :
> 
> mdd_thread_info.mti_xattr_buf
> 
> is there to be used when reading/changing a XATTR. If I code an helper 
> method in MDD which needs to manipulate an XATTR, this field looks 
> really interesting. But, I do not know for sure that all callers we not 
> be using it? So the only solution is that I verify each of them are not 
> using it (and their callers)? And latter, if someone add a new way to 
> call my method, he must looks for what stuff i'm using in the 
> thread_info to be sure it does not conflict with what he currently uses? 
> This seems a lot of code to check...

Well, c'est la vie. :-) I just ran a grep and found that ->mti_xattr_buf is used by 

    * mdd_create(), 

    * __mdd_lma_get(),

    * __mdd_lma_set(),

    * mdd_acl_chmod() and

    * mdd_check_acl().

Generally, foo_thread_info fields can only be used within the foo layer and one modifying it, is assumed to understand the layer.

Thank you,
Nikita. 

> 
> 
> And so, how can I chose that this field is relevant to be added to 
> thread_info instead of allocating/freeing it inside my function?
> 
> 
> Thanks for both your help
> 
> Aurélien
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