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

[Suren Baghdasaryan <surenb@google.com>]

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

> >               kthread_init_delayed_work(&group->poll_work,
> >                               psi_poll_work);
> >               rcu_assign_pointer(group->poll_kworker, kworker);
> > --
> > 2.22.0.709.g102302147b-goog
> >
>
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