Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle

From: Dario Faggioli <dfaggioli@suse.com>
To: "Jürgen Groß" <jgross@suse.com>, "Jan Beulich" <jbeulich@suse.com>
Cc: Charles Arnold <carnold@suse.com>, Glen <glenbarney@gmail.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Tomas Mozes <hydrapolic@gmail.com>, Sarah Newman <srn@prgmr.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle
Date: Thu, 12 Mar 2020 17:59:56 +0100	[thread overview]
Message-ID: <54fc0576bd7aa1ad213f5a07fef856c894e2d65f.camel@suse.com> (raw)
In-Reply-To: <21abe34e-54f0-70e3-d2f1-cf6984b5773a@suse.com>


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On Thu, 2020-03-12 at 17:00 +0100, Jürgen Groß wrote:
> On 12.03.20 16:26, Jan Beulich wrote:
> > On 12.03.2020 14:44, Dario Faggioli wrote:
> > > --- a/xen/common/sched/credit2.c
> > > +++ b/xen/common/sched/credit2.c
> > > @@ -234,7 +234,7 @@
> > >    * units does not consume credits, and it must be lower than
> > > whatever
> > >    * amount of credit 'regular' unit would end up with.
> > >    */
> > > -#define CSCHED2_IDLE_CREDIT          (-(1U<<30))
> > > +#define CSCHED2_IDLE_CREDIT          INT_MIN
> > 
> > The title saying "lower than", is "equal" actually fine? Looking
> > at e.g. runq_insert() I'm getting the impression it's not.
> > 
> > Looking at t2c_update() I'm also getting the impression that
> > there's UB when the subtraction underflows. After all, if
> > -1 << 30 wasn't small enough a value, I don't see why -1 << 31
> > would be.
> 
> Yes, I'd limit svc->credit in t2c_update() to CSCHED2_IDLE_CREDIT+1
> 
Right, after having spoken with George as well, I guess I'll go for
something like that (by imposing some boundaries to `val`, in that
function, I think).

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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<<This happens because _I_ choose it to happen!>> (Raistlin Majere)


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