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

Suren Baghdasaryan
2018-12-18T20:29:40Z

On Tue, Dec 18, 2018 at 11:18 AM Joel Fernandes <joelaf@google.com> wrote:
>
> On Tue, Dec 18, 2018 at 9:58 AM 'Suren Baghdasaryan' via kernel-team
> <kernel-team@android.com> wrote:
> >
> > 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.
>
> Suren, please avoid top-posting to LKML.

Sorry, did that by accident.

> Also I was going to say the same thing, just usecs only is better.

Thanks for the input.

> thanks,
>
> - Joel

> > 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.
> >
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