On Wed, Apr 07, 2021 at 08:34:24PM +0200, Peter Zijlstra <peterz@infradead.org> wrote:
> IMO as long as cgroups have that tasks file, you get to support people
> using it. That means that tasks joining your cgroup need to 'inherit'
> cgroup properties.
The tasks file is consequence of binding this to cgroups, I'm one step
back. Why to make "core isolation" a cgroup property?

(I understand this could help "visualize" what the common domains are if
cgroups were the only API but with prctl the structure can be
arbitrarily modified anyway.)


> Given something like:
> 
>         R
>        / \
>       A   B
>          / \
>         C   D
Thanks for the example. 

> B group can set core_sched=1 and then all its (and its decendants) tasks
> get to have the same (group) cookie and cannot share with others.
The same could be achieved with the first task of group B allocating its
new cookie which would be inherited in its descednants.

> If however B is a delegate and has a subgroup D that is security
> sensitive and must not share core resources with the rest of B, then it
> can also set D.core_sched=1, such that D (and its decendants) will have
> another (group) cookie.
If there is such a sensitive descendant task, it could allocate a new
cookie (same way as the first one in B did).

> On top of this, say C has a Real-Time tasks, that wants to limit SMT
> interference, then it can set a (task/prctl) cookie on itself, such that
> it will not share the core with the rest of the tasks of B.
(IIUC, in this particular example it'd be redundant if B had no inner
tasks since D isolated itself already.)
Yes, so this is again the same pattern as the tasks above have done.

> In that scenario the D subtree is a restriction (doesn't share) with the
> B subtree.
This implies D's isolation from everything else too, not just B's
members, no?

> And all of B is a restriction on all its tasks, including the Real-Time
> task that set a task cookie, in that none of them can share with tasks
> outside of B (including system tasks which are in R), irrespective of
> what they do with their task cookie.
IIUC, the equivalent restriction could be achieved with the PTRACE-like
check in the prctl API too (with respectively divided uids).

I'm curious whether the cgroup API actually simplifies things that are
possible with the clone/prctl API or allows anything that wouldn't be
otherwise possible.

Regards,
Michal