Re: [PATCH 1/1] psi: do not require setsched permission from the trigger creator

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 30, 2019 at 10:44:51AM -0700, Suren Baghdasaryan wrote:
> On Tue, Jul 30, 2019 at 1:11 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Jul 29, 2019 at 06:33:10PM -0700, Suren Baghdasaryan wrote:
> > > When a process creates a new trigger by writing into /proc/pressure/*
> > > files, permissions to write such a file should be used to determine whether
> > > the process is allowed to do so or not. Current implementation would also
> > > require such a process to have setsched capability. Setting of psi trigger
> > > thread's scheduling policy is an implementation detail and should not be
> > > exposed to the user level. Remove the permission check by using _nocheck
> > > version of the function.
> > >
> > > Suggested-by: Nick Kralevich <nnk@google.com>
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  kernel/sched/psi.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > > index 7acc632c3b82..ed9a1d573cb1 100644
> > > --- a/kernel/sched/psi.c
> > > +++ b/kernel/sched/psi.c
> > > @@ -1061,7 +1061,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
> > >                       mutex_unlock(&group->trigger_lock);
> > >                       return ERR_CAST(kworker);
> > >               }
> > > -             sched_setscheduler(kworker->task, SCHED_FIFO, &param);
> > > +             sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param);
> >
> > ARGGH, wtf is there a FIFO-99!! thread here at all !?
> 
> We need psi poll_kworker to be an rt-priority thread so that psi

There is a giant difference between 'needs to be higher than OTHER' and
FIFO-99.

> notifications are delivered to the userspace without delay even when
> the CPUs are very congested. Otherwise it's easy to delay psi
> notifications by running a simple CPU hogger executing "chrt -f 50 dd
> if=/dev/zero of=/dev/null". Because these notifications are

So what; at that point that's exactly what you're asking for. Using RT
is for those who know what they're doing.

> time-critical for reacting to memory shortages we can't allow for such
> delays.

Furthermore, actual RT programs will have pre-allocated and locked any
memory they rely on. They don't give a crap about your pressure
nonsense.

> Notice that this kworker is created only if userspace creates a psi
> trigger. So unless you are using psi triggers you will never see this
> kthread created.

By marking it FIFO-99 you're in effect saying that your stupid
statistics gathering is more important than your life. It will preempt
the task that's in control of the band-saw emergency break, it will
preempt the task that's adjusting the electromagnetic field containing
this plasma flow.

That's insane.

I'm going to queue a patch to reduce this to FIFO-1, that will preempt
regular OTHER tasks but will not perturb (much) actual RT bits.