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

[James Morse]

Hi Peter,

On 21/10/2022 13:42, Peter Newman wrote:
> 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.

Makes sense to me - do you want to send a patch to fix it?


Thanks,

James