Date: Fri, 12 Jun 2009 10:10:09 -0600 From: Scott Long <scottl@samsco.org> To: Julian Elischer <julian@elischer.org> Cc: src-committers@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org>, svn-src-all@FreeBSD.org, Marko Zec <zec@FreeBSD.org>, svn-src-head@FreeBSD.org, Alexander Kabaev <kabaev@gmail.com> Subject: Re: svn commit: r194012 - in head: . sys/netgraph sys/sys Message-ID: <4A327DE1.9040806@samsco.org> In-Reply-To: <4A31F9BE.6000405@elischer.org> References: <200906111650.n5BGonnn053446@svn.freebsd.org> <20090611190140.GE2642@garage.freebsd.pl> <200906112123.02105.zec@freebsd.org> <4A3168A0.2090308@elischer.org> <20090611202757.7cb0dad5@kan.dnsalias.net> <4A31F9BE.6000405@elischer.org>
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Julian Elischer wrote: > Alexander Kabaev wrote: >> On Thu, 11 Jun 2009 13:27:12 -0700 >> Julian Elischer <julian@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. > > You've already had the author of the osd code say that osd is designed to have little overhead. Maybe it's time to at least give it a try and make some measurements? Scott
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