lkml.org 
[lkml]   [2007]   [Aug]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: allow non root users to set io priority "idle" ?
Andi Kleen wrote:
> dragoran <drago01@gmail.com> writes:
>
>
>> Its possible to set the io priority using tools like ionice and syscalls.
>> But this works only as root.
>> Why can't a non root user change the priority of his processes to idle?
>> I understand that realtime priority requires root but why idle?
>> For instance the beagle trys to set its priority to idle but fails
>> because it does not run as root.
>> Any reason for this that I have missed? I can't think of a case where
>> setting the io priority to idle would have a negative impact for other
>> users or the whole system.
>>
>
> Very low priority can starve others when it holds some kernel resource
> needed by another task.
>
> Consider three tasks: one very low priority, one high priority: Low
> priority task holds some kernel resource, middle task eats as much CPU
> as it gets; high priority task wants to get the resource. High
> priority will need to wait for low running, which could take a long
> time. With true SCHED_IDLE (i believe the current implementation is
> not true) this could be never or at least a very long time.
>
> There are ways to defend against this problem (known as priority inheritance),
> but the kernel doesn't do them consistently. The same issue could also
> happen for user space managed resources.
>
> For IO I suppose the same could happen too. e.g. low priority
> task wants to write out a page and keeps it locked until the IO
> is finished. High priority task wants to access the page and has
> to wait until it is unlocked. Middle task generates an endless
> stream of IO that makes the idle priority writeout never finish.
couldn't this be fixed by bumping idle tasks to middle while they hold a
pagelock?
ex:
task A - hp
task B - mp
task C - lp (idle)
task C locks a page -> becomes a mp (middle prio) task until its finished
task C unlocks the page -> back to idle

> In general idle priorities are quite risky, even for root.
>
> -Andi
>
>

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-08-06 12:29    [W:0.081 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site