TCP prequeue goal is to defer processing of incoming packets
to user space thread currently blocked in a recvmsg() system call.
Intent is to spend less time processing these packets on behalf
of softirq handler, as softirq handler is unfair to normal process
scheduler decisions, as it might interrupt threads that do not
even use networking.
Current prequeue implementation has following issues :
1) It only checks size of the prequeue against sk_rcvbuf
It was fine 15 years ago when sk_rcvbuf was in the 64KB vicinity.
But we now have ~8MB values to cope with modern networking needs.
We have to add sk_rmem_alloc in the equation, since out of order
packets can definitely use up to sk_rcvbuf memory themselves.
2) Even with a fixed memory truesize check, prequeue can be filled
by thousands of packets. When prequeue needs to be flushed, either
from sofirq context (in tcp_prequeue() or timer code), or process
context (in tcp_prequeue_process()), this adds a latency spike
which is often not desirable.
I added a fixed limit of 32 packets, as this translated to a max
flush time of 60 us on my test hosts.
Also note that all packets in prequeue are not accounted for tcp_mem,
since they are not charged against sk_forward_alloc at this point.
This is probably not a big deal.
Note that this might increase LINUX_MIB_TCPPREQUEUEDROPPED counts,
which is misnamed, as packets are not dropped at all, but rather pushed
to the stack (where they can be either consumed or dropped)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
0cef6a4c34