Re: [Xen-devel] BUG: sched=credit2 crashes system when using cpupools

[Steven Haigh <netwiz@xxxxxxxxx>]

On Thursday, 13 September 2018 1:11:20 AM AEST Dario Faggioli wrote:
> On Thu, 2018-08-30 at 18:49 +1000, Steven Haigh wrote:
> > On 2018-08-30 18:33, Jan Beulich wrote:
> > > Anyway - as Jürgen says, something for the scheduler
> > > maintainers to look into.
> 
> Ok, I'm back.
> 
> > Yep - I just want to confirm that we tested this in BOTH NUMA
> > configurations - and credit2 crashed on both.
> > 
> > I switched back to sched=credit, and it seems to work as expected:
> > # xl cpupool-list
> > Name               CPUs   Sched     Active   Domain count
> > Pool-node0          12    credit       y          3
> > Pool-node1          12    credit       y          0

Hi Dario,

I'll try to clarify below.
 
> Wait, in a previous message, you said: "A machine where we could get
> this working every time shows". Doesn't that mean creating a separate
> pool for node 1 works with both Credit and Credit2, if the node has
> memory?
> 
> I mean, trying to clarifying, my understanding is that you have to
> systems:
> 
> system A: node 1 has *no* memory
> system B: both node 0 and node 1 have memory
> 
> Creating a Credit pool with pcpus from node 1 always work on both
> systems.

Correct. With the credit scheduler, the pool split worked correctly on both 
systems.
 
> OTOH, when you try to create a Credit2 pool with pcpus from node 1,
> does it always crash on both systems, or does it work on system B and
> crashes on system A ?

Both systems crashed when using credit2. We originally thought this was due to 
the different memory layout between the two systems. This bit turned out to 
not matter as both systems crashed in the same way.

> I do have a NUMA box with RAM in both nodes (so similar to system B).
> Last time I checked, what you're trying to do worked there, pretty much
> with any scheduler combination, but I'll recheck.
> 
> I don't have a box similar to system A. I'll try to remove some of the
> RAM from that NUMA box, and check what happens.

In theory, if you reproduce what we did, it should crash anyway. The RAM 
layout shouldn't matter.

We changed the scheduler in the grub line as 'sched=credit2'. Then did the 
split. I didn't try changing the Dom0 to boot with credit, but making the 
pools credit2.

-- 
Steven Haigh

📧 netwiz@xxxxxxxxx       💻 https://www.crc.id.au
📞 +61 (3) 9001 6090    📱 0412 935 897

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