Re: [RFC PATCH 00/13] Core scheduling v5

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Jun 26, 2020 at 11:10:28AM -0400, Joel Fernandes wrote:
> On Fri, Jun 26, 2020 at 10:36:01AM -0400, Vineeth Remanan Pillai wrote:
> > On Thu, Jun 25, 2020 at 9:47 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > On Thu, Jun 25, 2020 at 4:12 PM Vineeth Remanan Pillai
> > > <vpillai@digitalocean.com> wrote:
> > > [...]
> > > > TODO lists:
> > > >
> > > >  - Interface discussions could not come to a conclusion in v5 and hence would
> > > >    like to restart the discussion and reach a consensus on it.
> > > >    - https://lwn.net/ml/linux-kernel/20200520222642.70679-1-joel@joelfernandes.org
> > >
> > > Thanks Vineeth, just want to add: I have a revised implementation of
> > > prctl(2) where you only pass a TID of a task you'd like to share a
> > > core with (credit to Peter for the idea [1]) so we can make use of
> > > ptrace_may_access() checks. I am currently finishing writing of
> > > kselftests for this and post it all once it is ready.
> > >
> > Thinking more about it, using TID/PID for prctl(2) and internally
> > using a task identifier to identify coresched group may have
> > limitations. A coresched group can exist longer than the lifetime
> > of a task and then there is a chance for that identifier to be
> > reused by a newer task which may or maynot be a part of the same
> > coresched group.
> 
> True, for the prctl(2) tagging (a task wanting to share core with
> another) we will need some way of internally identifying groups which does
> not depend on any value that can be reused for another purpose.
> 
> [..]
> > What do you think about having a separate cgroup for coresched?
> > Both coresched cgroup and prctl() could co-exist where prctl could
> > be used to isolate individual process or task and coresched cgroup
> > to group trusted processes.
> 
> This sounds like a fine idea to me. I wonder how Tejun and Peter feel about
> having a new attribute-less CGroup controller for core-scheduling and just
> use that for tagging. (No need to even have a tag file, just adding/removing
> to/from CGroup will tag).

+Tejun

thanks,

 - Joel


> > > However a question: If using the prctl(2) on a CGroup tagged task, we
> > > discussed in previous threads [2] to override the CGroup cookie such
> > > that the task may not share a core with any of the tasks in its CGroup
> > > anymore and I think Peter and Phil are Ok with.  My question though is
> > > - would that not be confusing for anyone looking at the CGroup
> > > filesystem's "tag" and "tasks" files?
> > >
> > Having a dedicated cgroup for coresched could solve this problem
> > as well. "coresched.tasks" inside the cgroup hierarchy would list all
> > the taskx in the group and prctl can override this and take it out
> > of the group.
> 
> We don't even need coresched.tasks, just the existing 'tasks' of CGroups can
> be used.
> 
> > > To resolve this, I am proposing to add a new CGroup file
> > > 'tasks.coresched' to the CGroup, and this will only contain tasks that
> > > were assigned cookies due to their CGroup residency. As soon as one
> > > prctl(2)'s the task, it will stop showing up in the CGroup's
> > > "tasks.coresched" file (unless of course it was requesting to
> > > prctl-share a core with someone in its CGroup itself). Are folks Ok
> > > with this solution?
> > >
> > As I mentioned above, IMHO cpu cgroups should not be used to account
> > for core scheduling as well. Cpu cgroups serve a different purpose
> > and overloading it with core scheduling would not be flexible and
> > scalable. But if there is a consensus to move forward with cpu cgroups,
> > adding this new file seems to be okay with me.
> 
> Yes, this is the problem. Many people use CPU controller CGroups already for
> other purposes. In that case, tagging a CGroup would make all the entities in
> the group be able to share a core, which may not always make sense. May be a
> new CGroup controller is the answer (?).
> 
> thanks,
> 
>  - Joel
>