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

From: Joel Fernandes <joelaf@google.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tejun Heo <tj@kernel.org>,
	lizefan@huawei.com, Jens Axboe <axboe@kernel.dk>,
	dennis@kernel.org, Dennis Zhou <dennisszhou@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jonathan Corbet <corbet@lwn.net>,
	cgroups@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Cc: Android Kernel" <kernel-team@android.com>
Subject: Re: [PATCH 6/6] psi: introduce psi monitor
Date: Tue, 18 Dec 2018 11:18:21 -0800	[thread overview]
Message-ID: <CAJWu+oor7SzQVgo7SDKFcJS-BX+Oexj2_x_uWGsC8W=ErNGoNw@mail.gmail.com> (raw)
In-Reply-To: <CAJuCfpGQKQ9oVKdVeLNQHY2+2XTjLXb6VHDcJKAUCtSxvd68wQ@mail.gmail.com>

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.

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

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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