Re: LEON SMP

[Daniel Hellstrom <daniel@gaisler.com>]

David Miller wrote:

>From: Daniel Hellstrom <daniel@gaisler.com>
>Date: Tue, 26 Oct 2010 19:50:36 +0200
>
>  
>
>>The LEON do not have internal timers as some CPUs does, it has
>>one/multiple General Purpose TIMERs on the Processor Local Bus. On
>>single-CPU/SMP systems the first Timer is used for System Clock, and
>>on SMP systems timer two is also used to generate a simultaneous IRQ
>>on all CPUs for profiling etc. (leon_percpu_timer_interrupt()). On the
>>quad-core SMP system I discovered that since the per-cpu timer is
>>generated at the same frequency (and almost simultaneously) as the
>>System Clock Timer. I have made a patch that uses only one Timer for
>>SMP systems, the Timer generates a per-cpu tick as before, however on
>>CPU0 the handler_irq() is also called after profiling has been done,
>>this is to handle the System Clock Tick. I seems to work successfully,
>>and it saves me HZ interrupts per second and a Timer instance. What is
>>you opinion about that? Is it possible to use the same timer for
>>System Clock and for per-cpu profiling etc.?
>>    
>>
>
>You only need to generate one timer interrupt per-cpu, and the kernel
>generically decides to run the global timer actions (jiffies update,
>etc.) on a choosen cpu, transparently, in the per-cpu periodic timer
>code.
>
>  
>
That is interesting, I didn't even think about that. So then I can even 
remove the extra call to handler_irq() from within the per-cpu timer IRQ 
handler, and I will probably have to fix some code in the System Clock 
Timer setup as well. I will have to investigate this further then.

So actually it is bad to make 5 timer IRQs per tick on a quad core 
system, and one of them is calling handler_irq. But I can see that the 
system clock is progressing in the correct pace, unless my watch is bad 
:) I will get back to this issue later on.

Thank you,

Daniel