Re: [PATCH v3]Export cpu topology by sysfs

[Nathan Lynch <ntl@pobox.com>]

(sorry for the delay in response)

Yanmin Zhang wrote:
> Here is version 3. Based on Nathan's comments, the main changes are:
> 1) Drop thread id, so the first 2 patches of version 2 are dropped;

Thanks.

> 2) Set consistent default values for the 4 attributes.
> 

<snip>

> If one architecture wants to support this feature, it just needs to
> implement 4 defines, typically in file include/asm-XXX/topology.h.
> The 4 defines are:
> #define topology_physical_package_id(cpu)
> #define topology_core_id(cpu)
> #define topology_thread_siblings(cpu)
> #define topology_core_siblings(cpu)
> 
> The type of **_id is int.
> The type of siblings is cpumask_t.
> 
> To be consistent on all architectures, the 4 attributes should have
> deafult values if their values are unavailable. Below is the rule.
> 1) physical_package_id: If cpu has no physical package id, -1 is the
> default value.
> 2) core_id: If cpu doesn't support multi-core, its core id is 0.

Why not -1 as with the physical package id?  0 could be a valid core
id.

> 3) thread_siblings: Just include itself, if the cpu doesn't support
> HT/multi-thread.
> 4) core_siblings: Just include itself, if the cpu doesn't support
> multi-core and HT/Multi-thread.

Really, I think the least confusing interface would not export those
attributes which are not relevant for the system.  E.g. if the system
isn't multi-core, you don't see core_id and core_siblings attributes.

Failing that, let's at least have consistent, unambiguous values for
the ids which are not applicable.

<snip>
> +static int __cpuinit topology_cpu_callback(struct notifier_block *nfb,
> +		unsigned long action, void *hcpu)
> +{
> +	unsigned int cpu = (unsigned long)hcpu;
> +	struct sys_device *sys_dev;
> +
> +	sys_dev = get_cpu_sysdev(cpu);
> +	switch (action) {
> +		case CPU_ONLINE:
> +			topology_add_dev(sys_dev);
> +			break;
> +		case CPU_DEAD:
> +			topology_remove_dev(sys_dev);
> +			break;
> +	}
> +	return NOTIFY_OK;
> +}

I still oppose this bit.  I want the attributes there for powerpc even
for offline cpus -- the topology information (if obtainable, which it
is on powerpc) is useful regardless of the cpu's online state.  The
attributes should appear when a cpu is made present, and go away when
a cpu is removed.

This week I'll try to do an implementation for powerpc.