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

[Michal Hocko <mhocko@suse.com>]

On Mon 27-02-23 11:50:48, Suren Baghdasaryan wrote:
> On Mon, Feb 27, 2023 at 11:11 AM Michal Hocko <mhocko@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@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@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.
-- 
Michal Hocko
SUSE Labs