Re: allow non root users to set io priority "idle" ?

[dragoran]

Andi Kleen wrote:
> dragoran <drago01@xxxxxxxxx> 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
>
>   

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