linux-kernel - Re: Why does ionice(1) ban the user to set back to 'none' class?

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Message-ID: <4FDFDFCE.9010307@gmail.com>
Date:	Tue, 19 Jun 2012 10:11:26 +0800
From:	"gnehzuil.lzheng@...il.com" <gnehzuil.lzheng@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
CC:	Rob Landley <rob@...dley.net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, util-linux@...r.kernel.org
Subject: Re: Why does ionice(1) ban the user to set back to 'none' class?

[CC'd to util-linux mailing list]

On 06/18/2012 09:28 PM, Jens Axboe wrote:

> On 06/18/2012 01:48 PM, Zheng Liu wrote:
>> Hi Jens,
>>
>> I meet a problem when I use ionice(1) to adjust a process's io priority.
>> I do the following operations:
>>
>> $ ionice -p${pid}
>> none: prio 0
>> $ ionice -p${pid} -c2 -n4
>> $ ionice -p${pid}
>> best-effort: prio 4
>> $ ionice -p${pid} -c0 -n0
>> $ ionice -p${pid}
>> best-effort: prio 0
>>
>> So I cannot set scheduling class back to 'none'.  If I call ioprio_set(2)
>> directly, it will be fine.  But if I use ionice(1), I cannot change it.  I
>> read the docs about ionice in [1].  I notice this code:
>>
>> switch (ioprio_class) {
>> 	case IOPRIO_CLASS_NONE:
>> 		ioprio_class = IOPRIO_CLASS_BE;
>>                                ^^^^^^^^^^^^^^^^
>>                                *It means that we cannot set back to none.*
>> 		break;
>> 	case IOPRIO_CLASS_RT:
>> 	case IOPRIO_CLASS_BE:
>> 		break;
>> 	case IOPRIO_CLASS_IDLE:
>> 		ioprio = 7;
>> 		break;
>> 	default:
>> 		printf("bad prio class %d\n", ioprio_class);
>> 		return 1;
>> }
>>
>> My question is why we need to ban the user to set back to 'none'.  Is there
>> some reasons?  Thank you.
> 
> The kernel code does allow it, so it's only in the tool that this
> restriction exists. I don't think we have a particularly good reason to
> have it there. This is a tools question, though, not a kernel issue. I
> suggest you take it up with the util-linux crew. I'm fine with removing
> this restriction from ionice, especially since you could just roll your
> own that did it.
> 


Thanks for your reply.  Hopefully we can get some feedbacks from
util-linux crew.

Regards,
Zheng
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