svn commit: r194012 - in head: . sys/netgraph sys/sys

[Julian Elischer]

Alexander Kabaev wrote:
> On Thu, 11 Jun 2009 13:27:12 -0700
> Julian Elischer <julian at elischer.org> wrote:
> 
>> Marko Zec wrote:
>>> On Thursday 11 June 2009 21:01:40 Pawel Jakub Dawidek wrote:
>>>> On Thu, Jun 11, 2009 at 04:50:49PM +0000, Marko Zec wrote:
>>>>> Author: zec
>>>>> Date: Thu Jun 11 16:50:49 2009
>>>>> New Revision: 194012
>>>>> URL: http://svn.freebsd.org/changeset/base/194012
>>>>>
>>>>> Log:
>>>>>   Introduce a mechanism for detecting calls from outbound path of
>>>>> the network stack when reentering the inbound path from netgraph,
>>>>> and force queueing of mbufs at the outbound netgraph node.
>>>>>
>>>>>   The mechanism relies on two components.  First, in netgraph
>>>>> nodes where outbound path of the network stack calls into
>>>>> netgraph, the current thread has to be appropriately marked using
>>>>> the new NG_OUTBOUND_THREAD_REF() macro before proceeding to call
>>>>> further into the netgraph topology, and unmarked using the
>>>>>   NG_OUTBOUND_THREAD_UNREF() macro before returning to the caller.
>>>>>   Second, netgraph nodes which can potentially reenter the network
>>>>>   stack in the inbound path have to mark their inbound hooks using
>>>>>   NG_HOOK_SET_TO_INBOUND() macro.  The netgraph framework will
>>>>> then detect when there is a danger of a call graph looping back
>>>>> from outbound to inbound path via netgraph, and defer handing off
>>>>> the mbufs to the "inbound" node to a worker thread with a clean
>>>>> stack.
>>>>>
>>>>>   In this first pass only the most obvious netgraph nodes have
>>>>> been updated to ensure no outbound to inbound calls can occur.
>>>>> Nodes such as ng_ipfw, ng_gif etc. should be further examined
>>>>> whether a potential for outbound to inbound call looping exists.
>>>>>
>>>>>   This commit changes the layout of struct thread, but due to
>>>>>   __FreeBSD_version number shortage a version bump has been
>>>>> omitted at this time, nevertheless kernel and modules have to be
>>>>> rebuilt.
>>>> Are you sure Marko that you can't use sys/sys/osd.h instead of
>>>> adding yet another field to the thread structure? Netgraph is
>>>> optional component and optional components could take advantage of
>>>> allocating stuff they need dynamically. The OSD (Object-Specific
>>>> Data) KPI is designed for use by optional components - you can add
>>>> your data to a thread, you can get it when you want and OSD will
>>>> call your callback when thread dies, so you can clean up.
>>>>
>>>> Maybe you can't, but it's worth checking.
>>> Hmm how much locking overhead do osd_set() / osd_get() methods
>>> introduce?  We have to bump the refcount on each entry to netgraph,
>>> and then check it potentially on each hop to next ng node, and
>>> finally drop the refcount when done with the function call into
>>> netgraph.  Accessing td_ng_outbound directly via curthread is as
>>> cheap as it gets performancewise as it requires no locking
>>> whatsoever...
>> I would add that I suspect that we may end up using it in other
>> places as well outside of netgraph.
>>
> 
> When that happens then per-thread field can be revisited again. Blowing
> the side of major kernel structure for the sake of subsystem is
> unused by 90%+ percent of users is little too drastic IMHO.
> 
> I do second Pawel's opinion that you should look at osd for the time
> being. After all it was invented for just this reason.

And I beg to dissagree.

Firstly this is not the first field to be put in these structures for 
a single module.

Secondly, the overhead of doing it in the manner suggested would
be quite noticeable I think, certainly a drain on what could be
a fast-path for some packet processing.