Re: SCHED_DEADLINE as user

Hi Juri,

On Thu, 23 Aug 2018 12:23:50 +0200
Juri Lelli [off-list ref] wrote:

quoted

sorry for the late reply (I am just back from vacations), and thanks
for cc-ing me.  

Welcome back! :-)

Thanks :)

quoted

I see you opened a bug on github about this, so I am going to add
more details there,  

I use github issues to keep track of things, but I guess mailing list
discussions are to be preferred though (so that more people can
potentially follow).

Ok

quoted

but basically I think that in order to use SCHED_DEADLINE as
non-root we need to:
1) Disable the boosting mechanism (not the inheritance, just the
"soft enforcement behaviour" of tasks holding mutexes)  

But then what is a sane inheritance mechanism?

In my understanding (please correct me if I am wrong), this is an
orthogonal issue: if I understand well, the issue preventing non-root
usage of SCHED_DEADLINE is that a task inheriting a dl entity is not
throttled (when the current runtime rrives to 0, the deadline is
postponed, but the task stays schedulable). So, I think that removing
this behaviour should allow to use SCHED_DEADLINE without starving
other tasks...

Then, there is the issue about the deadline and runtime to inherit. And
I agree that this is important (and the solution is not easy), but you
have this issue even if you use the current "dl_boosted" behaviour...
No?

Walk the chain and find
the next potential deadline to inherit for the current boosted (still
runtime enforced) task before throttling it? Not sure it's going to be
any easier than the proxy solution. :-/

Right; this is not easy... But I think it is not related with the issue
we are discussing (if I understand this email thread well). Yes, it has
to be fixed, but it does not prevent non-root usage. Or am I missing
something?

quoted

2) Implement some mechanism to limit the amount of dl bandwidth a
user can allocate to itself (I think the cgroup-based approach we
   discussed some time ago should be OK... If I remember well, you
even had a patch implementing it :)  

I think most (all?) distributions today run with CONFIG_RT_GROUP_SCHED
disabled, we should also think about a solution that doesn't rely on
that interface.

I guess CONFIG_RT_GROUP_SCHED is often disabled because it ends up
changing the "traditional" SCHED_{RR/FIFO} behaviour. So, maybe the
solution is to have a different dl_{runtime,period} interface in
control groups (enabled by CONFIG_DL_GROUP_SCHED :).
CONFIG_DL_GROUP_SCHED does not change the scheduling behaviour, but
only the admission test... So, enabling it could be safer than enabling
CONFIG_RT_GROUP_SCHED.

Maybe the existing system wide sched_rt_{period,
runtime}_us are enough?

I do not know... A cgroup-based interface looks more powerful (and not
so difficult to implement... :), and would allow the sysadmin to decide
which users can use SCHED_DEADLINE, how much SCHED_DEADLINE bandwidth
can each user/group use, etc...


				Luca