Re: [Xen-devel] CPU Lockup bug with the credit2 scheduler

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

On 17/02/2020 20:58, Sarah Newman wrote:
> On 1/7/20 6:25 AM, Alastair Browne wrote:
>>
>> CONCLUSION
>>
>> So in conclusion, the tests indicate that credit2 might be unstable.
>>
>> For the time being, we are using credit as the chosen scheduler. We
>> are booting the kernel with a parameter "sched=credit" to ensure that
>> the correct scheduler is used.
>>
>> After the tests, we decided to stick with 4.9.0.9 kernel and 4.12 Xen
>> for production use running credit1 as the default scheduler.
> 
> One person CC'ed appears to be having the same experience, where the credit2 
> scheduler leads to lockups (in this case in the domU, not the dom0) under 
> relatively heavy load. It seems possible they may have the same root cause.
> 
> I don't think there are, but have there been any patches since the 4.13.0 
> release which might have fixed problems with credit 2 scheduler? If not, 
> what would the next step be to isolating the problem - a debug build of Xen 
> or something else?
> 
> If there are no merged or proposed fixes soon, it may be worth considering 
> making the credit scheduler the default again until problems with the 
> credit2 scheduler are resolved.
> 
> Thanks, Sarah
> 
> 

Hi Sarah / Alastair,

I can only provide my n=1 (OK, I'm running a bunch of boxes, some of which 
pretty over-committed CPU wise), 
but I haven't seen any issues (lately) with credit2.

I did take a look at Alastair Browne's report your replied to 
(https://lists.xen.org/archives/html/xen-devel/2020-01/msg00361.html)
and I do see some differences:
    - Alastair's machine has multiple sockets, my machines don't.
    - It seems Alastair's config is using ballooning ? 
(dom0_mem=4096M,max:16384M), for me that has been a source of trouble in the 
past, so my configs don't.
    - kernel's tested are quite old (4.19.67 (latest upstream is 4.19.104), 
4.9.189 (latest upstream is 4.9.214)) and no really new kernel is tested
      (5.4 is available in Debian backport for buster). 
    - Alastair, are you using pv, hvm or pvh guests? The report seems to miss 
the Guest configs (I'm primarily using PVH, and few HVM's, no PV except for 
dom0) ?

Any how, could be worthwhile to test without ballooning, and test a recent 
kernel to rule out an issue with (missing) kernel backports.

--
Sander

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