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packet.7: Status in PACKET_RX_RING is actually a bit mask 

Stefan:
I'm playing with the PACKET_RX_RING option to packet sockets and I
noticed that the status field from the tpacket_hdr structures
inside the mmaped buffer is actually a bit mask. I'm testing with
4Gbps traffic rates and sometimes I get status=5. I'm using Ubuntu
12.04 with a 3.8.0-29 kernel, and at least from the code (I got it
using "sudo apt-get source linux-image-$(uname -r)") it seems the
5 means TP_STATUS_USER | TP_STATUS_LOSING. The 3.8.0 kernel code
has this line in tpacket_rcv():

    /*
     * LOSING will be reported till you read the stats,
     * because it's COR - Clear On Read.
     * Anyways, moving it for V1/V2 only as V3 doesn't need this
     * at packet level.
     */
            if (po->stats.tp_drops)
                    status |= TP_STATUS_LOSING;

Daniel:
Now to your question. It can easily be seen from the if_packet.h header
file http://lingrok.org/xref/linux-net-next/include/uapi/linux/if_packet.h#93
that TP_STATUS_* are individual bits that are set in tp_status field.

TP_STATUS_USER simply means a frame was received in the ring and is
ready for user space to be processed. If the field also indicates
TP_STATUS_LOSING then this means that there were packet drops in the
rx ring itself so a user knows it didn't get all traffic. This bit
is being reset on getsockopt() when querying PACKET_STATISTICS,
otherwise it stays. Drops occur when the ring buffer was not emptied
fast enough by user space (so no free slot with TP_STATUS_KERNEL), e.g.
due to high incoming traffic load. However, the current frame you're
reading that has TP_STATUS_USER|TP_STATUS_LOSING is fine by itself.

Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Carsten Andrich <carsten.andrich@tu-ilmenau.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Stefan Puiu 2014-04-24 11:25:19 +02:00 committed by Michael Kerrisk
parent 84f573db60
commit a29edaeef6
1 changed files with 4 additions and 4 deletions
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8
man7/packet.7
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@ -9,7 +9,7 @@
.\" .\"
.\" $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $ .\" $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $
.\" .\"
.TH PACKET 7 2014-04-23 "Linux" "Linux Programmer's Manual" .TH PACKET 7 2014-04-24 "Linux" "Linux Programmer's Manual"
.SH NAME .SH NAME
packet \- packet interface on device level packet \- packet interface on device level
.SH SYNOPSIS .SH SYNOPSIS
@ -364,9 +364,9 @@ The packet socket owns all slots with status
.BR TP_STATUS_KERNEL . .BR TP_STATUS_KERNEL .
After filling a slot, it changes the status of the slot to transfer After filling a slot, it changes the status of the slot to transfer
ownership to the application. ownership to the application.
During normal operation, the new status is During normal operation, the new status has the
.BR TP_STATUS_USER , .BR TP_STATUS_USER
to signal that a correctly received packet has been stored. bit set to signal that a received packet has been stored.
When the application has finished processing a packet, it transfers When the application has finished processing a packet, it transfers
ownership of the slot back to the socket by setting the status to ownership of the slot back to the socket by setting the status to
.BR TP_STATUS_KERNEL . .BR TP_STATUS_KERNEL .