[PATCH] psi: reduce min window size to 50ms

[Suren Baghdasaryan]

On Tue, Feb 28, 2023 at 5:50 AM Michal Hocko <mhocko at suse.com> wrote:
>
> On Mon 27-02-23 11:50:48, Suren Baghdasaryan wrote:
> > On Mon, Feb 27, 2023 at 11:11 AM Michal Hocko <mhocko at suse.com> wrote:
> > >
> > > On Mon 27-02-23 09:49:59, Suren Baghdasaryan wrote:
> > > > On Mon, Feb 27, 2023 at 5:34 AM Michal Hocko <mhocko at suse.com> wrote:
> > > > >
> > > > > On Fri 24-02-23 13:07:57, Suren Baghdasaryan wrote:
> > > > > > On Fri, Feb 24, 2023 at 4:47 AM Michal Hocko <mhocko at suse.com> wrote:
> > > [...]
> > > > > > > Btw. it seems that there is is only a limit on a single trigger per fd
> > > > > > > but no limits per user so it doesn't sound too hard to end up with too
> > > > > > > much polling even with a larger timeouts. To me it seems like we need to
> > > > > > > contain the polling thread to be bound by the cpu controller.
> > > > > >
> > > > > > Hmm. We have one "psimon" thread per cgroup (+1 system-level one) and
> > > > > > poll_min_period for each thread is chosen as the min() of polling
> > > > > > periods between triggers created in that group. So, a bad trigger that
> > > > > > causes overly aggressive polling and polling thread being throttled,
> > > > > > might affect other triggers in that cgroup.
> > > > >
> > > > > Yes, and why that would be a problem?
> > > >
> > > > If unprivileged processes are allowed to add new triggers then a
> > > > malicious process can add a bad trigger and affect other legit
> > > > processes. That sounds like a problem to me.
> > >
> > > Hmm, I am not sure we are on the same page. My argument was that the
> > > monitoring kernel thread should be bound by the same cpu controller so
> > > even if it was excessive it would be bound to the cgroup constrains.
> >
> > Right. But if cgroup constraints are violated then the psimon thread's
> > activity will be impacted by throttling. In such cases won't that
> > affect other "good" triggers served by that thread even if they are
> > using higher polling periods?
>
> That is no different from any other part of the workload running within
> the same cpu bound cgroup running overboard with the cpu consumption. I
> do not see why psimon or anything else should be any different.
>
> Actually the only difference here is that the psi monitoring is
> outsourced to a kernel thread which is running ourside of any constrains.
> I am not sure where do we stand with kernel thread cpu cgroup accounting
> and I suspect this is not a trivial thing to do ATM. Hence longer term
> plan.

Yeah, that sounds right.
In the meantime I think the prudent thing to do is to add
CAP_SYS_RESOURCE check for cgroup interface for consistency with
system-wide one. After that we can change the min period to be
anything more than 0 and let userspace privileged services implement
policies to limit trigger cpu consumption (might be via cpu
controller, limiting the number of triggers/their periods, etc).
Sudarshan, I'll post the CAP_SYS_RESOURCE change shortly and you can
follow up with the change to the min trigger period.
Thanks for the input folks!

> --
> Michal Hocko
> SUSE Labs