Subject: Re: [PATCH v2 2/11] Add PRRN Event Handler

From: Nathan Fontenot <[email-masked]>
Date: Fri, 05 Apr 2013 10:43:24 -0500

On 04/03/2013 10:34 PM, Paul Mackerras wrote:
> On Mon, Mar 25, 2013 at 01:52:32PM -0500, Nathan Fontenot wrote:
>> From: Jesse Larrew <[email-masked]>
>>
>> A PRRN event is signaled via the RTAS event-scan mechanism, which
>> returns a Hot Plug Event message "fixed part" indicating "Platform
>> Resource Reassignment". In response to the Hot Plug Event message,
>> we must call ibm,update-nodes to determine which resources were
>> reassigned and then ibm,update-properties to obtain the new affinity
>> information about those resources.
>>
>> The PRRN event-scan RTAS message contains only the "fixed part" with
>> the "Type" field set to the value 160 and no Extended Event Log. The
>> four-byte Extended Event Log Length field is repurposed (since no
>> Extended Event Log message is included) to pass the "scope" parameter
>> that causes the ibm,update-nodes to return the nodes affected by the
>> specific resource reassignment.
>>
>> This patch adds a handler in rtasd for PRRN RTAS events. The function
>> pseries_devicetree_update() (from mobility.c) is used to make the
>> ibm,update-nodes/ibm,update-properties RTAS calls. Updating the NUMA maps
>> (handled by a subsequent patch) will require significant processing,
>> so pseries_devicetree_update() is called from an asynchronous workqueue
>> to allow rtasd to continue processing events. Since we flush all work
>> on the queue before handling any new work there should only be one event
>> in flight of being handled at a time.
>             ^^ "of" is superfluous

will remove it.

> 
> In the worst case where PRRN events come close together in time, the
> flush_work will block for however long it takes to do this
> "significant processing", meaning that we're no better off using a
> workqueue.  Do we have any reason to think that these PRRN events will
> normally be widely spaced in time?  If so you should mention it in the
> patch description.

Yes. PRRN events can only be triggered from the HMC by an IBM tech who has
to actualy log into a customer system and initiate the PRRN event. There
is no method for a user to initiate a PRRN event. Given this is is safe
to assume that these events will be widely spaced in time.

> 
> Also, rtasd isn't actually a task, it's just a function that gets run
> via schedule_delayed_work_on() and re-schedules itself each time it
> runs.  Is there any deadlock possibility in calling flush_work from a
> work function?

I don't know of any but I will investigate.

Thanks for the feedback.
-Nathan