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SubjectRe: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature
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Ingo Molnar <mingo@elte.hu> writes:

> * Jack O'Quin <joq@io.com> wrote:
>
>> > i'm wondering, couldnt Jackd solve this whole issue completely in
>> > user-space, via a simple setuid-root wrapper app that does nothing else
>> > but validates whether the user is in the 'jackd' group and then keeps a
>> > pipe open to to the real jackd process which it forks off, deprivileges
>> > and exec()s? Then unprivileged jackd could request RT-priority changes
>> > via that pipe in a straightforward way. Jack normally gets installed as
>> > root/admin anyway, so it's not like this couldnt be done.
>>
>> Perhaps.
>>
>> Until recently, that didn't work because of the longstanding rlimits
>> bug in mlockall(). For scheduling only, it might be possible.
>>
>> Of course, this violates your requirement that the user not be able to
>> lock up the CPU for DoS. The jackd watchdog is not perfect.
>
> there is a legitimate fear that if it's made "too easy" to acquire some
> sort of SCHED_FIFO priority, that an "arm's race" would begin between
> desktop apps, each trying to set themselves to SCHED_FIFO (or SCHED_ISO)
> and advising users to 'raise the limit if they see delays' - just to get
> snappier than the rest.
>
> thus after a couple of years we'd end up with lots of desktop apps
> running as SCHED_FIFO, and latency would go down the drain again.

I wonder how Mac OS X and Windows deal with this priority escalation
problem? Is it real or only theoretical?
--
joq
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