On 09/10/2024 02:05, Jason Xing wrote:
On Wed, Oct 9, 2024 at 7:22 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
On Wed, Oct 9, 2024 at 2:44 AM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
Jason Xing wrote:
From: Jason Xing <kernelxing@xxxxxxxxxxx>
A few weeks ago, I planned to extend SO_TIMESTMAMPING feature by using
tracepoint to print information (say, tstamp) so that we can
transparently equip applications with this feature and require no
modification in user side.
Later, we discussed at netconf and agreed that we can use bpf for better
extension, which is mainly suggested by John Fastabend and Willem de
Bruijn. Many thanks here! So I post this series to see if we have a
better solution to extend.
This approach relies on existing SO_TIMESTAMPING feature, for tx path,
users only needs to pass certain flags through bpf program to make sure
the last skb from each sendmsg() has timestamp related controlled flag.
For rx path, we have to use bpf_setsockopt() to set the sk->sk_tsflags
and wait for the moment when recvmsg() is called.
As you mention, overall I am very supportive of having a way to add
timestamping by adminstrators, without having to rebuild applications.
BPF hooks seem to be the right place for this.
There is existing kprobe/kretprobe/kfunc support. Supporting
SO_TIMESTAMPING directly may be useful due to its targeted feature
set, and correlation between measurements for the same data in the
stream.
After this series, we could step by step implement more advanced
functions/flags already in SO_TIMESTAMPING feature for bpf extension.
My main implementation concern is where this API overlaps with the
existing user API, and how they might conflict. A few questions in the
patches.
Agreed. That's also what I'm concerned about. So I decided to ask for
related experts' help.
How to deal with it without interfering with the existing apps in the
right way is the key problem.
What I try to implement is let the bpf program have the highest
precedence. It's similar to RTO min, see the commit as an example:
commit f086edef71be7174a16c1ed67ac65a085cda28b1
Author: Kevin Yang <yyd@xxxxxxxxxx>
Date: Mon Jun 3 21:30:54 2024 +0000
tcp: add sysctl_tcp_rto_min_us
Adding a sysctl knob to allow user to specify a default
rto_min at socket init time, other than using the hard
coded 200ms default rto_min.
Note that the rto_min route option has the highest precedence
for configuring this setting, followed by the TCP_BPF_RTO_MIN
socket option, followed by the tcp_rto_min_us sysctl.
It includes three cases, 1) route option, 2) bpf option, 3) sysctl.
The first priority can override others. It doesn't have a good
chance/point to restore the icsk_rto_min field if users want to
shutdown the bpf program because it is set in
bpf_sol_tcp_setsockopt().
rto_min example is slightly different. With tcp_rto_min the doesn't
expect any data to come back to user space while for timestamping the
app may be confused directly by providing more data, or by not providing
expected data. I believe some hint about requestor of the data is needed
here. It will also help to solve the problem of populating sk_err_queue
mentioned by Martin.
