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

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Date:   Thu, 13 Sep 2018 06:51:38 +0100
From:   Brice Goglin <brice.goglin@...il.com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Jeremy Linton <jeremy.linton@....com>, rjw@...ysocki.net,
        linux-acpi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, vkilari@...eaurora.org
Subject: Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

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).

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 :)

Brice