Re: [PATCH 1/1] psi: remove 500ms min window size limitation for triggers

[Suren Baghdasaryan]

On Thu, Mar 2, 2023 at 5:58 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 01-03-23 11:34:03, Suren Baghdasaryan wrote:
> > Current 500ms min window size for psi triggers limits polling interval
> > to 50ms to prevent polling threads from using too much cpu bandwidth by
> > polling too frequently. However the number of cgroups with triggers is
> > unlimited, so this protection can be defeated by creating multiple
> > cgroups with psi triggers (triggers in each cgroup are served by a single
> > "psimon" kernel thread).
> > Instead of limiting min polling period, which also limits the latency of
> > psi events, it's better to limit psi trigger creation to authorized users
> > only, like we do for system-wide psi triggers (/proc/pressure/* files can
> > be written only by processes with CAP_SYS_RESOURCE capability). This also
> > makes access rules for cgroup psi files consistent with system-wide ones.
> > Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and
> > remove the psi window min size limitation.
> >
> > Suggested-by: Sudarshan Rajagopalan <quic_sudaraja@xxxxxxxxxxx>
> > Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@xxxxxxxxxxx/
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> with this to fix
> [...]
> > @@ -1278,8 +1277,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
> >       if (state >= PSI_NONIDLE)
> >               return ERR_PTR(-EINVAL);
> >
> > -     if (window_us < WINDOW_MIN_US ||
> > -             window_us > WINDOW_MAX_US)
> > +     if (window_us <= 0 || window_us > WINDOW_MAX_US)
> >               return ERR_PTR(-EINVAL);
>
> window_us is u32 sp the check for <= 0 doesn't make any sense.

Completely missed that. Will change to == 0 and post the new version.
window_us is later multiplied by NSEC_PER_USEC and then divided by
UPDATES_PER_WINDOW(10), so even the smallest value of 1 still results
in a positive poll_min_period. I think we should be fine with that
check.
Thanks!

>
> >
> >       /* Check threshold */
> > --
> > 2.40.0.rc0.216.gc4246ad0f0-goog
>
> --
> Michal Hocko
> SUSE Labs