Subject: Re: [PATCH] Fix interrupt distribution in ppc970

From: Milton Miller <[email-masked]>
Date: Tue, 10 Apr 2007 11:59:28 -0500

On Apr 10, 2007, at 2:06 AM, Michael Ellerman wrote:

> On Mon, 2007-04-09 at 14:27 +0530, Mohan Kumar M wrote:
>> On Wed, Mar 07, 2007 at 11:52:32AM +0100, Michael Ellerman wrote:
>>> There's already maxcpus in init/main.c, that would probably be 
>>> better,
>>> though still ugly.
>>>
>> Based on Mike's suggestions, I modified the patch. The attached patch
>> refers max_cpus variable to check whether the kernel is booted with
>> maxcpus=1 parameter and if maxcpus=1 is specified the patch assigns 
>> only
>> the current boot cpu to be the default distribution server.
>
> So the core of the problem is that if we haven't onlined all cpus then
> we can't use the default_distrib_server value given to us by firmware,
> because some of the cpus in that queue won't be online.
>
> We can detect this situation by comparing the number of cpus that are
> online vs the number that are present (not possible). This might even
> work if you boot with maxcpus=1 and then hotplug the rest in.
>
> How about this:
>
> Index: powerpc/arch/powerpc/platforms/pseries/xics.c
> ===================================================================
> --- powerpc.orig/arch/powerpc/platforms/pseries/xics.c
> +++ powerpc/arch/powerpc/platforms/pseries/xics.c
> @@ -167,7 +167,10 @@ static int get_irq_server(unsigned int v
>  		return default_server;
>
>  	if (cpus_equal(cpumask, CPU_MASK_ALL)) {
> -		server = default_distrib_server;
> +		if (num_online_cpus() == num_present_cpus())
> +			server = default_distrib_server;
> +		else
> +			server = default_server;
>  	} else {
>  		cpus_and(tmp, cpu_online_map, cpumask);

This means we are doing population counts of two masks, when we really
just care that they are the same.   How about using
cpus_equal(cpu_online_mask, cpu_present_mask)?

>
> @@ -415,7 +418,10 @@ static void xics_set_affinity(unsigned i
>
>  	/* For the moment only implement delivery to all cpus or one cpu */
>  	if (cpus_equal(cpumask, CPU_MASK_ALL)) {
> -		newmask = default_distrib_server;
> +		if (num_online_cpus() == num_present_cpus())
> +			newmask = default_distrib_server;
> +		else
> +			newmask = default_server;
>  	} else {
>  		cpus_and(tmp, cpu_online_map, cpumask);
>  		if (cpus_empty(tmp))

This shows how close these two functions are.  The difference is
what happens when cpus_empty is true -- we default in one and
ignore in the other.

How about adding another arg to get_server, that says to fail
or default for the empty case, with failure being -1?

I'll try to code this up, but it might be a day or two until i get
the time.

milton