Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

[Sudeep Holla <sudeep.holla@arm.com>]

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