On 11/23/11 17:42, Julien Ridoux wrote:
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
There are a few nits, but the patch implements what I envisaged, thanks
Julien.
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.
Even though it looks a bit gross, we would still add it at the end to
avoid gratuitously breaking binaries. We would then also add some
explicit padding in the struct to soak up the redundant space left in
between it and the second last struct member.
What is your favourite option?
FreeBSD parlance is to ask what colour you would like to paint the
bikeshed ;)
As I've never experienced the pain John refers to, I'll defer to the
wisdom of others on whether the proposed patch will create pain down the
road. I think it's ok, but if consensus is 8bytes per packet isn't going
to break the bank, I guess we just go for it - but I guess I am cautious
about this route as we can push a lot of packets per second through the
stack.
Cheers,
Lawrence
_______________________________________________
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"
