Re: SMTC support status in latest git head.

["Kevin D. Kissell" <kevink@paralogos.com>]

On 12/24/10 11:32 PM, Anoop P A wrote:
> On Fri, 2010-12-24 at 15:34 -0800, Kevin D. Kissell wrote:
>> Ah, well, at least we have a stackframe.h fix that preserves David's
>> performance tweak for the deeper pipelined processors.  In looking for
>> this, I did notice that someone did some modification to the SMTC clock
>> tick logic that I was skeptical had ever been tested.  If you've still
>> got that kernel binary handy, you might check to see if it boots with
>> maxtcs=1 maxvpes=1, maxtcs=2 maxvpes=1, and/or maxtcs=2 maxvpes=2.
> Yes I have tried with various combinations of tcs and vpes. with
> maxvpes=1 I can boot with a max of 4 TCS ( VPE0 has 4 TCs) .
> However setting maxpes=2 and maxtcs=2 hangs pretty early.
>
> Clock rate set to 600000000
> console [ttyS0] enabled
> Calibrating delay loop... 398.33 BogoMIPS (lpj=796672)
> pid_max: default: 32768 minimum: 301
> Mount-cache hash table entries: 512
> Limit of 2 VPEs set
> Limit of 2 TCs set
> TLB of 64 entry pairs shared by 2 VPEs
> VPE 0: TC 0, VPE 1: TC 1
> IPI buffer pool of 32 buffers
> CPU revision is: 00019548 ((null))
> TC 1 going on-line as CPU 1
> Brought up 2 CPUs
>
> One strange observation is with maxtcs=3 and maxvpes=2 kernel boots all
> the way.
>
> Again with maxtcs=5 and maxvpes=2 it hangs after switching to MIPS
> clocksource.
>
> I strongly suspect some issue with locking. I will dig the code early
> next week.
If locking is screwed up, I'd expect more problems with 4 TC "CPUs" in 
the same VPE. It also suggests that the basic distribution via local 
low-latency IPI within a VPE is functioning, but that something is 
broken in the cross-VPE evengt propagation.  I strongly suspect that 
your maxtcs=3, maxvpes=2 case would hang sooner or later, but by luck of 
the draw none of the init threads got scheduled on VPE 1 long enough to 
get stuck.

I note that there were some changes made under the rubric "MIPS: SMTC: 
Avoid queueing multiple reschedule IPIs" in October and November of last 
year that make me nervous.  I wouldn't have coded things that way 
myself, but they might be OK. Still, the first bisection I'd make if I 
was trouble-shooting this would be to roll back to just before they went in.

             Ho, ho, ho,

             Kevin K.