From: Dario Faggioli <dfaggioli@suse.com>
To: George Dunlap <George.Dunlap@citrix.com>
Cc: Juergen Gross <jgross@suse.com>,
Charles Arnold <carnold@suse.com>,
Tomas Mozes <hydrapolic@gmail.com>, Glen <glenbarney@gmail.com>,
Jan Beulich <jbeulich@suse.com>, Sarah Newman <srn@prgmr.com>,
xen-devel <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 18:03:45 +0100 [thread overview]
Message-ID: <7308c02ccc7d741b0c660aaa0110b71a61a887a3.camel@suse.com> (raw)
In-Reply-To: <2B668743-662D-4A34-9ADE-F699A7BABF8A@citrix.com>
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On Thu, 2020-03-12 at 14:45 +0000, George Dunlap wrote:
> > On Mar 12, 2020, at 1:44 PM, Dario Faggioli <dfaggioli@suse.com>
> > wrote:
> >
> > This patch makes Credit2 more robust to events like this, whatever
> > the cause is, and should hence be backported (as far as possible).
> >
> > Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
> > Reported-by: Glen <glenbarney@gmail.com>
> > Reported-by: Tomas Mozes <hydrapolic@gmail.com>
>
> Nit: The reported-by’s should be before the SoB (i.e., tags roughly
> in time order).
>
Ah, right! :-(
> I think this is a good change to make the algorithm more robust, so:
>
> Acked-by: George Dunlap <george.dunlap@citrix.com>
>
> But it seems like allowing a guest to rack up -2^63 credits is still
> a bad thing, and it would be nice to have some other backstop / reset
> mechanism.
>
I agree. FWIW, this is way it took me a while to get to the bottom of
this. I was assuming it was *entirely* a Credit2 specific issue (caused
by, e.g., something like I found and fixed with patch 2).
When I noticed that things were not exactly like that, I also realized
that we at least need to prevent --under any circumstance-- that idle
vCPUs are preferred over "regular" vCPUs, and even if I did consider
approaches like "compacting" the credits dynamic, I went straight for
the INT_MIN approach.
Considering how this thread is going, I guess we should actually push
this further, and limit the "credit swing".
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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next prev parent reply other threads:[~2020-03-12 17:04 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 13:44 [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits Dario Faggioli
2020-03-12 13:44 ` [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle Dario Faggioli
2020-03-12 13:55 ` Andrew Cooper
2020-03-12 14:40 ` George Dunlap
2020-03-12 15:10 ` Dario Faggioli
2020-03-12 14:58 ` Dario Faggioli
2020-03-12 14:45 ` George Dunlap
2020-03-12 17:03 ` Dario Faggioli [this message]
2020-03-12 15:26 ` Jan Beulich
2020-03-12 16:00 ` Jürgen Groß
2020-03-12 16:59 ` Dario Faggioli
2020-03-12 16:11 ` Dario Faggioli
2020-03-12 16:36 ` Jan Beulich
2020-03-12 13:44 ` [Xen-devel] [PATCH 2/2] xen: credit2: fix credit reset happening too few times Dario Faggioli
2020-03-12 15:08 ` [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits Roger Pau Monné
2020-03-12 17:02 ` Dario Faggioli
2020-03-12 17:59 ` Roger Pau Monné
2020-03-13 6:19 ` Dario Faggioli
2020-03-12 15:51 ` Jürgen Groß
2020-03-12 16:27 ` Andrew Cooper
2020-03-13 7:26 ` Dario Faggioli
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