linux-kernel - Re: [PATCH] of: introduce event tracepoints for dynamic device

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Message-Id: <5aa699db-255a-773b-7d60-3c6f05e88050@linux.vnet.ibm.com>
Date:   Wed, 19 Apr 2017 14:13:50 -0700
From:   Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        rostedt@...dmis.org, Ingo Molnar <mingo@...hat.com>,
        Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH] of: introduce event tracepoints for dynamic device_node
 lifecyle

On 04/19/2017 03:13 AM, Michael Ellerman wrote:
> Oliver O'Halloran <oohall@...il.com> writes:
> 
>> On Wed, Apr 19, 2017 at 2:46 AM, Rob Herring <robh+dt@...nel.org> wrote:
>>> On Mon, Apr 17, 2017 at 7:32 PM, Tyrel Datwyler
>>> <tyreld@...ux.vnet.ibm.com> wrote:
>>>> This patch introduces event tracepoints for tracking a device_nodes
>>>> reference cycle as well as reconfig notifications generated in response
>>>> to node/property manipulations.
>>>>
>>>> With the recent upstreaming of the refcount API several device_node
>>>> underflows and leaks have come to my attention in the pseries (DLPAR) dynamic
>>>> logical partitioning code (ie. POWER speak for hotplugging virtual and physcial
>>>> resources at runtime such as cpus or IOAs). These tracepoints provide a
>>>> easy and quick mechanism for validating the reference counting of
>>>> device_nodes during their lifetime.
>>>
>>> Not really relevant for this patch, but since you are looking at
>>> pseries and refcounting, the refcounting largely exists for pseries.
>>> It's also hard to get right as this type of fix is fairly common. It's
>>> now used for overlays, but we really probably only need to refcount
>>> the overlays or changesets as a whole, not at a node level. If you
>>> have any thoughts on how a different model of refcounting could work
>>> for pseries, I'd like to discuss it.
>>
>> One idea I've been kicking around is differentiating short and long
>> term references to a node.
> 
> I actually did this a long time ago, but balked at the size of the patch
> to do the conversion. Let me see if I can find it lying around ...

It would be interesting to revisit this, and toss around any other ideas
anyone may have.

-Tyrel

> 
> cheers
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