Strange ARC/Swap/CPU on yesterday's -CURRENT

[Don Lewis]

On  7 Apr, Justin Hibbits wrote:
> On Fri, Apr 6, 2018 at 10:25 AM, Justin Hibbits <chmeeedalf at gmail.com> wrote:
>> On Fri, Apr 6, 2018 at 10:08 AM, Mark Johnston <markj at freebsd.org> wrote:
>>> On Fri, Apr 06, 2018 at 12:47:14AM +0000, Justin Hibbits wrote:
>>>> My powerpc64 embedded machine is virtually unusable since these vm changes.
>>>> I tried setting vfs.zfs.arc_free_target as suggested, and that didn't help
>>>> at all. Eventually the machine hangs and just gets stuck in vmdaemon, with
>>>> many processes in wait channel btalloc.
>>>
>>> You don't really have the same symptoms that Don is reporting.
>>
>> Okay.  I latched onto the thread because it seemed similar.
>>
>>> Threads being stuck in btalloc implies a KVA shortage. So:
>>> - What do you mean by "stuck in vmdaemon"?
>>
>> The machine hangs, and my ssh sessions get killed.  I can't do
>> anything at the console except break into kdb.  When I do, the running
>> thread is always vmdaemon.
>>
>>> - Which commits specifically cause problems? Does reverting r329882 fix the
>>>   hang?
>>
>> I'll try reverting it and report back.  Thankfully I can buildkernel
>> successfully on the machine before it hangs.  Can't do more than that,
>> though.
> 
> I reverted back to r329881, and successfully built world.  Updated to
> r329882 and it got stuck with processes in btalloc.
> 
> I've seen other reports of r329882 causing problems on powerpc64 as
> well, with other bizarre behaviors.

I initially missed that this was on powerpc.  I believe that arch is a
bit odd with characters unsigned by default and arithmentic shifts
always being unsigned.  Things like that could mess up the algorithm,
but I didn't see anything suspicious in the r329882 diff.