On 23/11/2011, at 1:00 AM, Lawrence Stewart wrote: > On 11/23/11 00:30, John Baldwin wrote: >> On Monday, November 21, 2011 2:28:10 am Lawrence Stewart wrote: >>> On 11/21/11 17:18, Julien Ridoux wrote: >>>> >>>> On 21/11/2011, at 4:39 PM, Lawrence Stewart wrote: >>>> >>>>> On 11/21/11 16:12, Ben Kaduk wrote: >>>>>> On Sun, Nov 20, 2011 at 11:17 PM, Lawrence >>>>>> Stewart<lstew...@freebsd.org> wrote: >>>>>>> Author: lstewart Date: Mon Nov 21 04:17:24 2011 New Revision: >>>>>>> 227778 URL: http://svn.freebsd.org/changeset/base/227778 >>>>>>> >>>>>>> Log: - When feed-forward clock support is compiled in, change >>>>>>> the BPF header to contain both a regular timestamp obtained >>>>>>> from the system clock and the current feed-forward ffcounter >>>>>>> value. This enables new possibilities including >>>>>> >>>>>> Is it really necessary to make the ABI dependent on a kernel >>>>>> configuration option? This causes all sorts of headaches if >>>>>> loadable modules ever want to use that ABI, something that we >>>>>> just ran into with vm_page_t and friends and had a long thread on >>>>>> -current about. >>>>> >>>>> Fair question. Julien, if pcap and other consumers will happily >>>>> ignore the new ffcount_stamp member in the bpf header, is there any >>>>> reason to conditionally add the ffcounter into the header struct? >>>> >>>> It is a valid question indeed. The feedback I have received so far >>>> was to not have the feed-forward clock support be a default kernel >>>> configuration option. What follows is based on this assumption. >>>> >>>> The commit (r227747) introduces sysctl that are conditioned by the >>>> same "FFCLOCK" kernel configuration option. If a loadable module >>>> tests for the presence of this sysctl, it will know if the >>>> ffcount_stamp member is available. Is it too much of a hack? >>>> >>>> Alternatively, if the ffcounter is added to the bpf header >>>> unconditionally, the ffcount_stamp member can be set to 0. Loadable >>>> modules will then see a consistent ABI but will retrieve a >>>> meaningless value. >>>> >>>> I am not sure which option makes more sense, any preference? >>> >>> If I understand the issues correctly, I think the appropriate path >>> forward is to remove the conditional change to the bpf header and have >>> ffcount_stamp become a permanent member of the struct. We'll just leave >>> the member uninitialised in the !FFCLOCK case. This change will make the >>> patch un-MFCable, but I think that's ok. >>> >>> As to the issue of how a kernel module would detect if it's being loaded >>> into a FFCLOCK enabled kernel, why wouldn't we expect modules to >>> "#include opt_ffclock.h" and conditionally compile code based on FFCLOCK >>> being defined? Is there a use case for run-time (as opposed to >>> compile-time) module detection of feed-forward clock capabilities? >> >> Think of standalone modules that are not built as part of a kernel (e.g. >> 3rd party device drivers). In general we should avoid having structures >> change size for kernel options, especially common structures. It just adds >> lots of pain and suffering and complexity. We are stuck with it for PAE on >> i386 (which causes pain), and for LOCK_PROFILING (but that is sufficiently >> rare and expensive it seems to be ok). I think 8 bytes for bpf packet is >> not sufficiently expensive to justify the extra headache. Just always leave >> the new field in. > > hmm... Julien almost has a patch finished which accomplishes what my most > recent email in this thread describes. Julien, I suggest we get it finished > and follow up to this thread with a pointer to the patch for people to look > at. If there's still a strong feeling that what it does is going to bring > pain we can do away with the new BPF_FFCOUNTER config option and have the bpf > header struct just grow by 8 bytes. > > Stay tuned...
Thanks all for the feedback. With some delay, I have a patch against r227871 that implements what Lawrence proposed. You can find it here: http://www.cubinlab.ee.unimelb.edu.au/~jrid/patches/ffclock-bpf-header-r227871.patch I have tested this under a few typical scenario, it works as expected but already brings some headaches (hence the long delay mentioned above :-)). I thought a bit more of user cases. I believe many of them call for having both feed-forward counter and its conversion in second be present in the BPF header. For example, this allows to have absolute packet departure/arrival times (as per usual), but also provides the opportunity to compute inter-arrival times accurately using the difference clock. There are other examples I can think of, and if one believe the feed-forward clock approach becomes more popular, such usages will be more and more common. Assuming the BPF header grows by 8 bytes independent of any kernel option, I admit that the current implementation is a bit ugly. The BPF structure is not nicely packed and looks clunky. Ideally, the feed-forward counter should be placed just below the bh_tstamp member, but this would require libpcap and all ports depending on it to be recompiled after this change. What is your favourite option? Julien_______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
