Re: [RFD] resctrl: reassigning a running container's CTRL_MON group

[Peter Newman]

On Thu, Oct 20, 2022 at 12:39 PM Peter Newman <peternewman@xxxxxxxxxx> wrote:
>
> On Wed, Oct 19, 2022 at 3:58 PM James Morse <james.morse@xxxxxxx> wrote:
> > The devil is in the detail, I'm not sure how it serialises with a fork()ing process, I'd
> > hope to do better than relying on the kernel walking the list of processes a lot quicker
> > than user-space can.
>
> I wasn't planning to do it any more optimally than the rmdir
> implementation today when looking for all tasks impacted by a
> CLOSID/RMID deletion.

This is probably a separate topic, but I noticed this when looking at how rmdir
moves tasks to a new closid/rmid...

In rdt_move_group_tasks(), how do we know that a task switching in on another
CPU will observe the updated closid and rmid values soon enough?

Even on x86, without an smp_mb(), the stores to t->closid and t->rmid could be
reordered with the task_curr(t) and task_cpu(t) reads which follow. The original
description of this scenario seemed to assume that accesses below would happen
in program order:

    WRITE_ONCE(t->closid, to->closid);
    WRITE_ONCE(t->rmid, to->mon.rmid);

    /*
     * If the task is on a CPU, set the CPU in the mask.
     * The detection is inaccurate as tasks might move or
     * schedule before the smp function call takes place.
     * In such a case the function call is pointless, but
     * there is no other side effect.
     */
    if (IS_ENABLED(CONFIG_SMP) && mask && task_curr(t))
         cpumask_set_cpu(task_cpu(t), mask);

If the task concurrently switches in on another CPU, the code above may not
observed that it's running, and the CPU running the task may not have observed
the updated rmid and closid yet, so it could continue with the old rmid/closid
and not get interrupted.

-Peter