Re: [PATCH v1] arch_topology: Make cpu_capacity sysfs node as ready-only

[Sudeep Holla]

On Wed, Mar 06, 2019 at 08:57:53PM +0530, Lingutla Chandrasekhar wrote:
> If user updates any cpu's cpu_capacity, then the new value is going to
> be applied to all its online sibling cpus. But this need not to be correct
> always, as sibling cpus (in ARM, same micro architecture cpus) would have
> different cpu_capacity with different performance characteristics.
> So updating the user supplied cpu_capacity to all cpu siblings
> is not correct.
>
> And another problem is, current code assumes that 'all cpus in a cluster
> or with same package_id (core_siblings), would have same cpu_capacity'.
> But with commit '5bdd2b3f0f8 ("arm64: topology: add support to remove
> cpu topology sibling masks")', when a cpu hotplugged out, the cpu
> information gets cleared in its sibling cpus. So user supplied
> cpu_capacity would be applied to only online sibling cpus at the time.
> After that, if any cpu hot plugged in, it would have different cpu_capacity
> than its siblings, which breaks the above assumption.
>
> So instead of mucking around the core sibling mask for user supplied
> value, use device-tree to set cpu capacity. And make the cpu_capacity
> node as read-only to know the assymetry between cpus in the system.
>

Acked-by: Sudeep Holla <sudeep.holla@xxxxxxx>

IIRC this was added for 2 possibilities though I don't completely agree
no one had any objections(including me though I wonder how/why I missed
to notice it now, anyways it's too late)

1. For systems that don't provide this information via device-tree/any
   firmware though that's the highly recommended way. With more complex
   topologies in horizon, I can't think of fetching/deducing this
   information *correctly* in any other sane way.

2. For some sort of tuning(avoid rebuild and reboot), but that's
   questionable as this is not a software characteristic. It's more
   like deriving hardware characteristics using software experiments.
   So, for me, we can compare this with some hardware latencies we have
   like CPU idle entry/exit latencies. They are tuned but not in
   production kernels. So if there's a case for adding this back as
   write capable sysfs, I would prefer that in debugfs and this sysfs
   is read-only ABI.

Hope that helps.

--
Regards,
Sudeep