Re: [PATCH 6/6] psi: introduce psi monitor

[Suren Baghdasaryan <surenb@google.com>]

Current design supports only whole percentages and if userspace needs
more granularity then it has to use usecs.
I agree that usecs cover % usecase and "threshold * win / 100" is
simple enough for userspace to calculate. I'm fine with changing to
usecs only.

On Tue, Dec 18, 2018 at 9:30 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Tue, Dec 18, 2018 at 11:46:22AM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 17, 2018 at 05:21:05PM -0800, Suren Baghdasaryan wrote:
> > > On Mon, Dec 17, 2018 at 8:22 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > > How well has this thing been fuzzed? Custom string parser, yay!
> > >
> > > Honestly, not much. Normal cases and some obvious corner cases. Will
> > > check if I can use some fuzzer to get more coverage or will write a
> > > script.
> > > I'm not thrilled about writing a custom parser, so if there is a
> > > better way to handle this please advise.
> >
> > The grammar seems fairly simple, something like:
> >
> >   some-full = "some" | "full" ;
> >   threshold-abs = integer ;
> >   threshold-pct = integer, { "%" } ;
> >   threshold = threshold-abs | threshold-pct ;
> >   window = integer ;
> >   trigger = some-full, space, threshold, space, window ;
> >
> > And that could even be expressed as two scanf formats:
> >
> >  "%4s %u%% %u" , "%4s %u %u"
> >
> > which then gets your something like:
> >
> >   char type[5];
> >
> >   if (sscanf(input, "%4s %u%% %u", &type, &pct, &window) == 3) {
> >       // do pct thing
> >   } else if (sscanf(intput, "%4s %u %u", &type, &thres, &window) == 3) {
> >       // do abs thing
> >   } else return -EFAIL;
> >
> >   if (!strcmp(type, "some")) {
> >       // some
> >   } else if (!strcmp(type, "full")) {
> >       // full
> >   } else return -EFAIL;
> >
> >   // do more
>
> We might want to drop the percentage notation.
>
> While it's somewhat convenient, it's also not unreasonable to ask
> userspace to do a simple "threshold * win / 100" themselves, and it
> would simplify the interface spec and the parser.
>
> Sure, psi outputs percentages, but only for fixed window sizes, so
> that actually saves us something, whereas this parser here needs to
> take a fractional anyway. The output is also in decimal notation,
> which is necessary for granularity. And I really don't think we want
> to add float parsing on top of this interface spec.
>
> So neither the convenience nor the symmetry argument are very
> compelling IMO. It might be better to just not go there.