linux-kernel - Re: [PATCH] scsi_execute_async() should add to the tail of the queue

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Message-ID: <4587C04E.10307@monatomic.org>
Date:	Tue, 19 Dec 2006 12:34:54 +0200
From:	Dan Aloni <da-x@...atomic.org>
To:	Arjan van de Ven <arjan@...rus.demon.nl>
CC:	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Mike Christie <michaelc@...wisc.edu>
Subject: Re: [PATCH] scsi_execute_async() should add to the tail of the queue

Arjan van de Ven wrote:
> On Tue, 2006-12-19 at 10:35 +0200, Dan Aloni wrote:
>   
>> Hello,
>>
>> scsi_execute_async() has replaced scsi_do_req() a few versions ago, 
>> but it also incurred a change of behavior. I noticed that over-queuing 
>> a SCSI device using that function causes I/Os to be starved from 
>> low-level queuing for no justified reason.
>>  
>> I think it makes much more sense to perserve the original behaviour 
>> of scsi_do_req() and add the request to the tail of the queue.
>>     
>
> Hi,
>
> some things should really be added to the head of the queue, like
> maintenance requests and error handling requests. Are you sure this is
> the right change? At least I'd expect 2 apis, one for a head and one for
> a "normal" queueing...
>   
Since a user of scsi_execute_async() would most likely want to have
control over this, it would be better to add a parameter and fix the
current users of the function.

However, if we take this route we might have duplicate code
across mid-layer drivers (sg, st, osst), because they may choose to
prioritize I/Os in similar ways.

So instead of adding a parameter, we can make scsi_execute_async()
decide for itself based on the SCSI command, with read/write I/Os
taking the lowest priority.

Suggestions?

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