On Tue, Jan 23, 2018 at 2:20 PM, Lawrence Brakmo <bra...@fb.com> wrote:
> On 1/23/18, 9:30 AM, "Yuchung Cheng" <ych...@google.com> wrote:
>
> The original patch that changes TCP's congestion control via eBPF only
> re-initializes the new congestion control, if the BPF op is set to an
> (invalid) value beyond BPF_SOCK_OPS_NEEDS_ECN. Consequently TCP will
>
> What do you mean by “(invalid) value”?
>
> run the new congestion control from random states.
>
> This has always been possible with setsockopt, no?
>
> This patch fixes
> the issue by always re-init the congestion control like other means
> such as setsockopt and sysctl changes.
>
> The current code re-inits the congestion control when calling
> tcp_set_congestion_control except when it is called early on (i.e. op <=
> BPF_SOCK_OPS_NEEDS_ECN). In that case there is no need to re-initialize
> since it will be initialized later by TCP when the connection is established.
>
> Otherwise, if we always call tcp_reinit_congestion_control we would call
> tcp_cleanup_congestion_control when the congestion control has not been
> initialized yet.
On Tue, Jan 23, 2018 at 2:20 PM, Lawrence Brakmo <bra...@fb.com> wrote:
> On 1/23/18, 9:30 AM, "Yuchung Cheng" <ych...@google.com> wrote:
>
> The original patch that changes TCP's congestion control via eBPF only
> re-initializes the new congestion control, if the BPF op is set to an
> (invalid) value beyond BPF_SOCK_OPS_NEEDS_ECN. Consequently TCP will
>
> What do you mean by “(invalid) value”?
>
> run the new congestion control from random states.
>
> This has always been possible with setsockopt, no?
>
> This patch fixes
> the issue by always re-init the congestion control like other means
> such as setsockopt and sysctl changes.
>
> The current code re-inits the congestion control when calling
> tcp_set_congestion_control except when it is called early on (i.e. op <=
> BPF_SOCK_OPS_NEEDS_ECN). In that case there is no need to re-initialize
> since it will be initialized later by TCP when the connection is established.
>
> Otherwise, if we always call tcp_reinit_congestion_control we would call
> tcp_cleanup_congestion_control when the congestion control has not been
> initialized yet.
Interesting. So I wonder if the symptoms we were seeing were due to
kernels that did not yet have this fix:
27204aaa9dc6 ("tcp: uniform the set up of sockets after successful
connection):
https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=27204aaa9dc67b833b77179fdac556288ec3a4bf
Before that fix, there could be TFO passive connections that at SYN time called:
tcp_init_congestion_control(child);
and then:
tcp_call_bpf(child, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB);
So that if the CC was switched in the
BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB handler then there would be no
init for the new module?
neal
