On 12/02/11 03:43, Jung-uk Kim wrote:
On Thursday 01 December 2011 10:11 am, Lawrence Stewart wrote:
On 11/30/11 05:09, Jung-uk Kim wrote:
On Tuesday 29 November 2011 11:13 am, Lawrence Stewart wrote:
On 11/30/11 00:05, Lawrence Stewart wrote:
On 11/28/11 14:59, Benjamin Kaduk wrote:
On Wed, 23 Nov 2011, Lawrence Stewart wrote:
On 11/23/11 17:42, Julien Ridoux wrote:
[snip]
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.
Since other people seem to be keeping quiet, I'll add that I'm
in favor of just always adding the 8 bytes per packet.
Julien and I discussed this at length today, and agree that for
head, we'll add the new bh_ffcounter member to the BPF header
unconditionally.
Thanks to you and John for the input.
I'm going to revert r227778 in order to start form a clean
slate, and add two separate patches. One will reintegrate
FFCLOCK support with BPF without breaking the ABI. A follow up
patch will bump the ffclock version and add the bh_ffcounter to
the bpf header (after the timestamp member). Then a final patch
will bump __FreeBSD_version and add a note to UPDATING about
recompiling to get kernel/world in sync, which should seal the
deal.
Here's the first of the patches:
http://people.freebsd.org/~lstewart/patches/misc/ffclock_bpf_int
act abi_10.x.r228130.patch
I only glanced at it but it looks very close to what I wanted to
suggest.
Final candidate patch is at:
http://people.freebsd.org/~lstewart/patches/misc/ffclock_bpf_intact
abi_10.x.r228180.patch
Assuming it passes the "make tinderbox" build I'm currently running
and no further input is received from interested parties, I plan to
commit it in ~10 hours.
Changes since the r228130 patch I sent previously:
- The new flags in bpf.h are added unconditionally so that they can
always be referenced at compile time and a decision made at runtime
as to whether a flag will be set or not. Using one of the new flags
when the kernel doesn't have FFCLOCK compiled in results in the
flag being ignored. An app should check for the existence of the
"ffclock" kernel feature or the "kern.sysclock" sysctl tree before
attempting to use the flags.
- This patch will hopefully be MFCed at some point, so I added a
CTASSERT to bpf.c to ensure that the ABI of structs bpf_hdr32,
bpf_hdr and bpf_xhdr remains intact when FFCLOCK is enabled and the
union of a ffcounter and struct timeval32/timeval/bpf_ts is
switched in.
- bpf_gettime() more comprehensively covers all the possible cases
of flag combination and does sensible things for each case (even
though some cases are rather silly).
- The snippet of code at the beginning of catchpacket() that was
manipulating the struct bintime derived from bpf_gettime() was
gross and has been removed in favour of selecting the right
{get}bin{up}time() function call in bpf_gettime().
I did that to reduce branching. Since you are introducing more
branches, it warrants a function pointer now.
I see, thanks for the explanation. Could you elaborate a bit more about
how you would implement the function pointer idea? I'm also curious in
the !FFCLOCK case just how much overhead having the 2-layer nested
if/else adds. I'm not an very optimisation savvy person, but I'm
wondering if it's actually worth micro-optimising this code.
My initial thoughts about your function pointer idea lead to adding a
function pointer in the bpf_d and setting it to the appropriate function
to get the timestamp from at bpf_d creation or ioctl time. Whilst I like
this idea, I can't see how it would work given that the various
functions involved in time/ffcounter stamp generation all have different
signatures.
We could have multiple variants of bpf_gettime() which each call the
appropriate underlying functions to generate the appropriate stamp.
Would add quite a lot of code but would reduce the overhead of calling
bpf_gettime() to an indirect function call + the underlying stamp
generation function call. This also solves the problem of multiple
function signatures. We would also need to add the function pointer to
the bpf_d struct which I guess breaks the ABI, something we're trying to
avoid with this patch as we want to MFC it.
Other ideas would be very welcome.
Cheers,
Lawrence
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