On Tue, Mar 13, 2018 at 09:38:41AM +0200, Artem Bityutskiy wrote: > On Tue, 2018-03-13 at 11:11 +0800, Dou Liyang wrote: > > I also > > met the situation that BIOS told to ACPI that it could support > > physical > > CPUs hotplug, But actually, there was no hardware slots in the > > machine. > > the ACPI tables like user inputs which should be validated when we > > use. > > This is exactly what happens on Skylake Xeon systems. When I check > dmesg or this file: > > /sys/devices/system/cpu/possible > > on 2S (two socket) and 4S (four socket) systems, I see the same number > 432. > > This number comes from ACPI MADT. I will speculate (did not see myself) > that 8S systems will report the same number as well, because of the > Skylake-SP (Scalable Platform) architecture. > > Number 432 is good for 8S systems, but it is way too large for 2S and > 4S systems - 4x or 2x larger than the theoretical maximum. > > I do not know why BIOSes have to report unrealistically high numbers, I > am just sharing my observation. > > So yes, Linux kernel's possible CPU count knowledge may be too large. > If we use that number to evenly spread IRQ vectors among the CPUs, we > end up with wasted vectors, and even bugs, as I observe on a 2S > Skylake. Then looks this issue need to fix by making possible CPU count accurate because there are other resources allocated according to num_possible_cpus(), such as percpu variables. Thanks, Ming
