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 <[email protected]> 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
