Re: Ryzen 4000 (Mobile) Softlocks/Micro-stutters

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Wed, 2020-12-23 at 10:59 +0100, Jan Beulich wrote:
> On 23.12.2020 00:04, Dylanger Daly wrote:
> > I think I've narrowed down what could be the issue.
> > 
> > I think disabling SMT on any AMD Zen 2 CPU is breaking Xen's
> > Credit2 scheduler, I can only test on AMD Ryzen 4000 based Mobile
> > CPUs, but I think this is what is causing issues with
> > softlocks/having to pin dom0 1 vcpu.
> 
> Dario,
> 
Hi, and thanks for bringing me in. :-)

> does this maybe ring any bells?
> 
Not really. :-(

Unfortunately, I don't think I have access to a Ryzen CPU (but I'll try
to look better).

I do have access to an EPYC2 (Rome) CPUs, i.e., an EPYC 7742 with 256
CPUs (128 cores x 2 threads). I have just tried booting Xen 4.14 there
and:

1) with all the 256 CPUs enabled (i.e., smt=1), Credit2 scheduler and 
   the default value (16) for sched_credit2_max_cpus_runqueue, the 
   system seem to work fine.

   There are 16 runqueues with 16 CPUs inside each of them, and they 
   seem to be constructed reasonably (siblings are in the same 
   runqueue, etc).

   I don't have a GUI on that box for checking whether mouse movement 
   are fluid, but I've run some basic tests from the terminal and 
   everything looks normal.

   Dom0 has 256 vCPUs and no pinning.

2) with only 128 CPUs (i.e., booting Xen with smt=0), Credit2 and  
   still 16 in sched_credit2_max_cpus_runqueue, it also seems to work 
   fine.

   There are again 16 runqueues, each one with 8 CPUs and the system 
   seems responsive enough.

   Dom0 has 128 vCPUs and no pinning.

I can try Credit as well, later, but if this is something CPU arch/gen
related, it seems to be a Ryzen rather than a Zen 2 thing...

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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