Re: [PATCH]sched: Isochronous class v2 for unprivileged soft rt scheduling

[Con Kolivas <kernel@kolivas.org>]

utz lehmann wrote:
> I had experimented with throttling runaway RT tasks. I use a similar
> accounting. I saw a difference between counting with or without the
> calling from fork. If i remember correctly the timeout expired too fast
> if the non-RT load was "while /bin/true; do :; done".
> With "while true; do :; done" ("true" is bash buildin) it worked good.
> But maybe it's not important in the real world.

It won't be relevant if we move to a sched_clock() based timesource 
anyway, which looks to be the next major development for this.

> If i understand sched_clock correctly it only has higher resolution if
> you can use tsc. In the non tsc case it's jiffies based. (On x86).
> I think you can easily fool a timer tick/jiffies based accounting and do
> a local DoS.

The same timer is used for accounting of SCHED_NORMAL tasks, so if you 
can work around that you can DoS the system with even the correct 
combination of SCHED_NORMAL tasks. When Ingo implemented sched_clock all 
the architectures slowly came on board. If I recall correctly the lowest 
resolution one still had microsecond accuracy which is more than enough 
given the time a context switch takes.

> Making SCHED_ISO privileged if you don't have a high resolution
> sched_clock is ugly.
> I really like the idea of a unprivileged SCHED_ISO but it has to be safe
> for a multi user system. And the kernel default should be safe for multi
> user.

Agreed; this is exactly what this work is about.

Cheers,
Con