Re: [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits

From: Dario Faggioli <dfaggioli@suse.com>
To: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Jürgen Groß" <jgross@suse.com>,
	xen-devel@lists.xenproject.org
Cc: Charles Arnold <carnold@suse.com>,
	Jan Beulich <jbeulich@suse.com>, Glen <glenbarney@gmail.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Tomas Mozes <hydrapolic@gmail.com>, Sarah Newman <srn@prgmr.com>
Subject: Re: [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits
Date: Fri, 13 Mar 2020 08:26:40 +0100	[thread overview]
Message-ID: <a6084dc725aedd9c6e5e25b7ec56a5e23dac2ccc.camel@suse.com> (raw)
In-Reply-To: <a4bddd49-6a9a-6f66-9154-745d0648b42b@citrix.com>

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On Thu, 2020-03-12 at 16:27 +0000, Andrew Cooper wrote:
> On 12/03/2020 15:51, Jürgen Groß wrote:
> > - Could it be the vcpu is busy for very long time in the
> > hypervisor?
> >   So either fighting with another vcpu for a lock, doing a long
> >   running hypercall, ...
> 
> Using watchdog=2 might catch that.  (There is a counting issue which
> I've not had time to fix yet, which makes the watchdog more fragile
> with
> a smaller timeout, but 2 should be ok.)
> 
Done (actually, I used "watchdog=1 watchdog_timeout=2"), both with and
without these patches applies.

I don't see any watchdog firing.

While I was there, I tried 1 too. Nothing either.

> > - The timer used is not reliable.
> > 
> > - The time base is not reliable (tsc or whatever is used for
> > getting
> >   the time has jumped 2 seconds into the future).
> 
> Worth instrumenting the TSC rendezvous for unexpectedly large jumps?
> 
Yes, we should look into that.

> > - System management mode has kicked in.
> 
> There are non-architectural SMI_COUNT MSRs (0x34 on Intel, can't
> remember AMD off the top of my head) which can be used to see if any
> have occurred, and this has proved useful in the past for debugging.
> 
I'll have to look up at how to check and dump these from where I need.
Let's see if I can do that... hints, code pointer or anything else you
may have handy would be much appreciated. :-)

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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