Re: [RFC PATCH 06/10] ARM: sched: Use device-tree to provide fast/slow CPU list for HMP

[Morten Rasmussen <Morten.Rasmussen@arm.com>]

On Thu, Oct 04, 2012 at 07:49:32AM +0100, Viresh Kumar wrote:
> On 22 September 2012 00:02,  <morten.rasmussen@arm.com> wrote:
> > From: Morten Rasmussen <morten.rasmussen@arm.com>
> >
> > We can't rely on Kconfig options to set the fast and slow CPU lists for
> > HMP scheduling if we want a single kernel binary to support multiple
> > devices with different CPU topology. E.g. TC2 (ARM's Test-Chip-2
> > big.LITTLE system), Fast Models, or even non big.LITTLE devices.
> >
> > This patch adds the function arch_get_fast_and_slow_cpus() to generate
> > the lists at run-time by parsing the CPU nodes in device-tree; it
> > assumes slow cores are A7s and everything else is fast. The function
> > still supports the old Kconfig options as this is useful for testing the
> > HMP scheduler on devices without big.LITTLE.
> 
> But this code is handling this case too at the end, with following logic:
> 
> > +       cpumask_setall(fast);
> > +       cpumask_clear(slow);
> 
> Am i missing something?
> 

The HMP setup can be defined using Kconfig or DT. If both fails, it will
set all cpus to be fast cpus and effectively disable SCHED_HMP. The
Kconfig option is kept to allow testing of alternative HMP setups
without having to change the DT or use DT at all which might be handy
for non-ARM platforms. I hope that answers you question.

> > This patch is reuse of a patch by Jon Medhurst <tixy@linaro.org> with a
> > few bits left out.
> 
> Then probably he must be the author of this commit? Also a SOB is required
> from him here.
> 

I don't know what the correct procedure is for this sort of partial
patch reuse. Since I didn't know better, I adopted Tixy's own reference
style that he used in one of his patches which is an extension of a
previous patch by me. I will of course fix it to follow normal procedure
if there is one.

> > Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> > ---
> >  arch/arm/Kconfig           |    4 ++-
> >  arch/arm/kernel/topology.c |   69 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 72 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index cb80846..f1271bc 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -1588,13 +1588,15 @@ config HMP_FAST_CPU_MASK
> >         string "HMP scheduler fast CPU mask"
> >         depends on SCHED_HMP
> >         help
> > -          Specify the cpuids of the fast CPUs in the system as a list string,
> > +          Leave empty to use device tree information.
> > +         Specify the cpuids of the fast CPUs in the system as a list string,
> >           e.g. cpuid 0+1 should be specified as 0-1.
> >
> >  config HMP_SLOW_CPU_MASK
> >         string "HMP scheduler slow CPU mask"
> >         depends on SCHED_HMP
> >         help
> > +         Leave empty to use device tree information.
> >           Specify the cpuids of the slow CPUs in the system as a list string,
> >           e.g. cpuid 0+1 should be specified as 0-1.
> >
> > diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c
> > index 26c12c6..7682e12 100644
> > --- a/arch/arm/kernel/topology.c
> > +++ b/arch/arm/kernel/topology.c
> > @@ -317,6 +317,75 @@ void store_cpu_topology(unsigned int cpuid)
> >                 cpu_topology[cpuid].socket_id, mpidr);
> >  }
> >
> > +
> > +#ifdef CONFIG_SCHED_HMP
> > +
> > +static const char * const little_cores[] = {
> > +       "arm,cortex-a7",
> > +       NULL,
> > +};
> > +
> > +static bool is_little_cpu(struct device_node *cn)
> > +{
> > +       const char * const *lc;
> > +       for (lc = little_cores; *lc; lc++)
> > +               if (of_device_is_compatible(cn, *lc))
> > +                       return true;
> > +       return false;
> > +}
> > +
> > +void __init arch_get_fast_and_slow_cpus(struct cpumask *fast,
> > +                                       struct cpumask *slow)
> > +{
> > +       struct device_node *cn = NULL;
> > +       int cpu = 0;
> > +
> > +       cpumask_clear(fast);
> > +       cpumask_clear(slow);
> > +
> > +       /*
> > +        * Use the config options if they are given. This helps testing
> > +        * HMP scheduling on systems without a big.LITTLE architecture.
> > +        */
> > +       if (strlen(CONFIG_HMP_FAST_CPU_MASK) && strlen(CONFIG_HMP_SLOW_CPU_MASK)) {
> > +               if (cpulist_parse(CONFIG_HMP_FAST_CPU_MASK, fast))
> > +                       WARN(1, "Failed to parse HMP fast cpu mask!\n");
> > +               if (cpulist_parse(CONFIG_HMP_SLOW_CPU_MASK, slow))
> > +                       WARN(1, "Failed to parse HMP slow cpu mask!\n");
> > +               return;
> > +       }
> > +
> > +       /*
> > +        * Else, parse device tree for little cores.
> > +        */
> > +       while ((cn = of_find_node_by_type(cn, "cpu"))) {
> > +
> > +               if (cpu >= num_possible_cpus())
> > +                       break;
> > +
> > +               if (is_little_cpu(cn))
> > +                       cpumask_set_cpu(cpu, slow);
> > +               else
> > +                       cpumask_set_cpu(cpu, fast);
> > +
> > +               cpu++;
> > +       }
> > +
> > +       if (!cpumask_empty(fast) && !cpumask_empty(slow))
> > +               return;
> > +
> > +       /*
> > +        * We didn't find both big and little cores so let's call all cores
> > +        * fast as this will keep the system running, with all cores being
> > +        * treated equal.
> > +        */
> > +       cpumask_setall(fast);
> > +       cpumask_clear(slow);
> > +}
> > +
> > +#endif /* CONFIG_SCHED_HMP */
> 
> All above calls to of_*() routines have dependency on CONFIG_OF
> 

It would be very easy to blame someone else here... :) I will fix it.

Thanks,
Morten

> --
> viresh
>