From: Keir Fraser <keir@xen.org>
To: Juergen Gross <juergen.gross@ts.fujitsu.com>
Cc: "Liu, Jinsong" <jinsong.liu@intel.com>,
xen-devel@lists.xensource.com, mark.langsdorf@amd.com
Subject: Re: [PATCH] use per-cpu variables in cpufreq
Date: Sat, 28 May 2011 08:52:49 +0100 [thread overview]
Message-ID: <CA066861.2E032%keir@xen.org> (raw)
In-Reply-To: <4DDFA72D.2060806@ts.fujitsu.com>
On 27/05/2011 14:29, "Juergen Gross" <juergen.gross@ts.fujitsu.com> wrote:
> On 05/27/11 15:11, Keir Fraser wrote:
>> On 27/05/2011 12:11, "Juergen Gross"<juergen.gross@ts.fujitsu.com> wrote:
>>
>>> The cpufreq driver used some local arrays indexed by cpu number. This patch
>>> replaces those arrays by per-cpu variables. The AMD and INTEL specific parts
>>> used different per-cpu data structures with nearly identical semantics.
>>> Fold the two structures into one by adding a generic architecture data item.
>> Xen's per-cpu data gets freed across cpu offline/online, whereas cpu-indexed
>> arrays of course do not. Will the cpufreq state be correctly handled across
>> offline/online if we switch to per-cpu vars?
>
> As far as I could see, yes. The data should only be used for cpus with
> a valid acpid->cpuid translation, which is created when a cpu is going
> online and destroyed when it is going offline again.
That simply isn't true. acpiid_to_apicid[] is populated during boot and
entries are never destroyed.
Specifically, my fear is that this data gets pushed into the hypervisor
once-only during dom0 boot (via XENPF_set_processor_pminfo). If it is freed
during processor offline, we lose it forever and have no power management
when/if a CPU is brought back online. Worse I suspect your patch as it is
will crash if some CPUs are offline during boot as you'll deference their
per_cpu area which doesn't actually exist unless a CPU is online. You can
test this for yourself by adding a maxcpus=1 boot parameter for Xen.
The folding of the Intel/AMD structures might still be interesting, and
probably belongs as a separate patch anyway.
Cc'ing Intel and AMD guys to confirm this.
-- Keir
> It would be nice, however, if the INTEL and/or AMD code owners could
> give an ack on this...
>
>
> Juergen
next prev parent reply other threads:[~2011-05-28 7:52 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-05-27 11:11 [PATCH] use per-cpu variables in cpufreq Juergen Gross
2011-05-27 13:11 ` Keir Fraser
2011-05-27 13:29 ` Juergen Gross
2011-05-28 7:52 ` Keir Fraser [this message]
2011-05-30 5:47 ` Juergen Gross
2011-05-30 9:45 ` Keir Fraser
2011-05-31 1:51 ` Tian, Kevin
2011-05-31 7:31 ` Keir Fraser
2011-05-31 7:37 ` Liu, Jinsong
2011-05-30 8:06 ` Tian, Kevin
2011-05-30 15:33 ` Liu, Jinsong
2011-06-10 19:00 ` Langsdorf, Mark
2011-06-14 9:04 ` Juergen Gross
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