On Thu, Sep 13, 2018 at 10:39:10AM +0100, James Morse wrote: > Hi Brice, > > On 13/09/18 06:51, Brice Goglin wrote: > > Le 12/09/2018 à 11:49, Sudeep Holla a écrit : > >>> Yes. Without this change, we hit the lscpu error in the commit message, > >>> and get zero output about the system. We don't even get information > >>> about the caches which are architecturally specified or how many cpus > >>> are present. With this change, we get what we expect out of lscpu (and > >>> also lstopo) including the cache(s) which are not architecturally > >>> specified. > >>> > >> lscpu and lstopo are so broken. They just assume everything on CPU0. > >> If you hotplug them out, you start seeing issues. So reading and file > >> that doesn't exist and then bail out on other essential info though they > >> are present, hmmm ... > > > > Can you elaborate? > > > > I am not sure cpu0 is supposed to be offlineable on Linux. There's no > > "online" file in /sys/devices/system/cpu/cpu0. That's why former lstopo > > doesn't like CPU0 being hotplugged out. We are actually making that case > > work for another non-standard corner case. But offlining "cpu0" this is > > considered "normal", somebody must add that missing "online" sysfs > > attribute for "cpu0" (change > > https://elixir.bootlin.com/linux/latest/source/drivers/base/cpu.c#L375). > > On x86 you can't normally offline CPU0, its something to do with certain > interrupts always being routed to CPU0, (oh, and hibernate). > You should be able to enable this behaviour with 'cpu0_hotplug' on the kernel > command line. > > (Kconfig's CONFIG_BOOTPARAM_HOTPLUG_CPU0 and CONFIG_DEBUG_HOTPLUG_CPU0 are also > worth a look) > > On arm64 at least, cpu0 is just like the others, and can be offlined. > Thanks James, for providing all the details. To add to the issues I spotted with lscpu/lstopo around topology, it ignores the updates to topology sibling masks when CPUs are hotplugged in and out. We have following in lscpu: add_summary_n(tb, _("Core(s) per socket:"), cores_per_socket ?: desc->ncores / desc->nsockets); Now when cores_per_socket = 1, (i.e when we don't have procfs entry), if ncores = (ncores_max - few_cpus_hotplugged_out), core(s) per socket will get computed as less than the actual number. IMO lscpu should be used only when all CPUs are online and it should have a warning when all cores are not online. > > By the way, did anybody actually see an error with lstopo when there's > > no "type" attribute for L3? I can't reproduce any issue, we just skip > > that specific cache entirely, but everything else appears. If you guys > > want to make that "no_cache" cache appear, I'll make it a Unified cache > > unless you tell me what to show :) IIUC, Jeffrey Hugo did see error as per his initial message: " This fixes the following lscpu issue where only the cache type sysfs file is missing which results in no output providing a poor user experience in the above system configuration. lscpu: cannot open /sys/devices/system/cpu/cpu0/cache/index3/type: No such file or directory " -- Regards, Sudeep [1] https://www.spinics.net/lists/arm-kernel/msg661101.html
