Re: [PATCH v2 0/3] x86/intel_rdt: task_work vs task_struct rmid/closid write race

[Reinette Chatre]

Hi Valentin,

On 11/25/2020 10:39 AM, Valentin Schneider wrote:
> On 25/11/20 17:20, Reinette Chatre wrote:
> > > > Until the queued work is run, the moved task runs with old (and even
> > > > invalid in the case when its original resource group has been removed)
> > > > closid and rmid.
> > >
> > > For a userspace task, that queued work should be run as soon as possible
> > > (& relevant). If said task is currently running, then task_work_add() will
> > > lead to an IPI;
> > > the other cases (task moving itself or not currently
> > > running) are covered by the return to userspace path.
> >
> > At this time the work is added with the TWA_RESUME flag so the running
> > task does not get a signal. I tried to follow the task_work_add() path
> > if there is a change to use TWA_SIGNAL instead and (I may have
> > misunderstanding) it seems to me that a sleeping task will be woken (if
> > it is TASK_INTERRUPTIBLE)? That is unnecessary. The goal of this work is
> > only to change the CPU register to indicate the active closid/rmid so it
> > is unnecessary to wake a process to do that, it only needs to be done
> > next time the task is scheduled in (which is already done with the
> > resctrl_sched_in() call in __switch_to()). If a task is not running all
> > that is needed is to change the closid/rmid in its task_struct to be
> > used next time it is scheduled in.
>
> The (default) TWA_RESUME ensures the targeted (userspace) task gets kicked
> if it is currently running, and doesn't perturb any CPU otherwise;
> see set_notify_resume() + exit_to_user_mode_loop() (or do_notify_resume()
> on arm64)

I missed that, thanks. The first issue is thus not a problem. Thank you 
very much for clearing this up. Queueing work for tasks that are not 
running remains unnecessary and simplifying this with a targeted 
smp_call_function addresses that (while also taking care of the other 
issues with using the queued work).

> > In the new solution, after updating closid/rmid in the task_struct, the
> > CPU register is updated via smp_call_function_single() on a CPU the task
> > is running. Nothing is done for tasks not running, next time they are
> > scheduled in the CPU's register will be updated to reflect the task's
> > closid/rmid. Moving to the smp_call_function_xxx() API would also bring
> > this update in line with how other register updates are already done in
> > resctrl.
> >
> > > Kernel threads however are a prickly matter because they quite explicitly
> > > don't have this return to userspace - they only run their task_work
> > > callbacks on exit. So we currently have to wait for those kthreads to go
> > > through a context switch to update the relevant register, but I don't
> > > see any other alternative that wouldn't involve interrupting every other
> > > CPU (the kthread could move between us triggering some remote work and its
> > > previous CPU receiving the IPI).
> >
> > This seems ok? In the new solution the closid/rmid would be updated in
> > task_struct and a smp_call_function_single() attempted on the CPU where
> > the kthread is running. If the kthread is no longer running at the time
> > the function is called the CPU register will not be changed.
>
> Right, if the update happens before triggering the remote work then that
> should all work. I was stuck thinking about keeping the update contained
> within the remote work itself to prevent any other races (i.e. patch 3).

Are you saying that the task_struct update as well as register update 
should both be done in the remote work? I think I may be 
misunderstanding though. Currently, with your entire series applied, the 
update to task_struct is done before the remote work is queued that only 
changes the register. The new solution would also first update the 
task_struct and then the remote work (this time with smp_call_function) 
will just update the register.

From what I understand your work in patch 3 would continue to be 
welcome with the new solution that will also update the task_struct and 
then trigger the remote work to just update the register.

> Anywho, that's enough speculation from me, I'll just sit tight and see what
> comes next!

Reinette

> > I assume
> > the kthread move would include a context switch that would result in the
> > register change (__switch_to()->resctrl_sched_in()) for the kthread to
> > run with its new closid/rmid after the move.


Reinette