On 09/26/2016 07:59 AM, Peter Zijlstra wrote: > On Mon, Sep 26, 2016 at 07:45:37AM -0400, Prarit Bhargava wrote: >>> But then code which reads those will have to *know* that those cores are >>> offline - otherwise it would be confused by what it is reading there. >> >> When offline, /sys/devices/system/cpuX/cpu/online is 0. The problem is that >> when online is 0, topology disappears so there is no way to determine _the >> location_ of the offline'd thread. > > As far as all that code is concerned, that CPU doesn't even have a > location anymore. > > While there might be some distinction between hotplug and physical > hotplug on the user API side (I really wouldn't know), there isn't on > the kernel side. > > Once you unplug a CPU, its _gone_. There isn't another hotplug operation > once you really take the CPU out.
There's a difference between soft remove (via sysfs) and a true hot remove operation (where the whole thing is physically removed). Soft remove only results in the processor being made "not available" to the scheduler. > > Offline means out gone, vamoosh. No, that is incorrect. The socket that contains the cores (and threads) is still plugged in. > > And it doesn't make sense to talk about the location of a resource > that's not there. > Again, it is _physically_ there. P.
