linux-kernel - Dynamic switching of io

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Date:	Fri, 12 Dec 2008 22:18:39 +0300
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Dynamic switching of io_context

Hello Jens,

In SCST (http://scst.sf.net) in some cases IO can be submitted 
asynchronously. This is possible for pass-through (i.e. using 
scsi_execute_async()) and BLOCKIO (i.e. using direct bio interface, see 
blockio_exec_rw() in 
http://scst.svn.sourceforge.net/viewvc/scst/trunk/scst/src/dev_handlers/scst_vdisk.c?revision=614&view=markup) 
backend. For them there's no need to have a per device pool of threads, 
one or more global thread(s) can perfectly do all the work. But it is 
very desirable for performance that all the IO is submitted in a 
dedicated IO context for each initiator (i.e. client), which originated 
it. I.e. commands from initiator 1 submitted in IO context IOC1, from 
initiator 2 - IOC2, etc. Most likely, the same approach would be very 
useful for NFS server as well.

To achieve that it is necessary to have a possibility to switch IO 
context of the threads on the fly. I tried to implement that (see the 
attached patch), but hit BUG_ON(!cic->dead_key) in cic_free_func(), when 
session for initiator with the corresponding IO context was being 
destroyed by scst_free_tgt_dev(). At that point it was guaranteed that 
there was no outstanding IO with this IO context.

So, I had to go to a more defensive approach to have for each pool of 
threads, including threads for async. IO, a dedicated IO context, which 
is currently implemented.

Could you advice please what was going wrong? What should I do to 
achieve what's desired?

Thanks,
Vlad

View attachment "tgt_dev_io_context.diff" of type "text/x-patch" (2247 bytes)