With config CONFIG_NETWORK_PHY_TIMESTAMPING enabled, the calls to
skb_clone_tx_timestamp() and skb_defer_rx_timestamp() are enabled (these
normally are empty inlined no-op functions). The overhead from what I
can see is very small, for example for the tx path:
static unsigned int classify(const struct sk_buff *skb)
{
if (likely(skb->dev && skb->dev->phydev &&
skb->dev->phydev->drv))
return ptp_classify_raw(skb);
else
return PTP_CLASS_NONE;
}
void skb_clone_tx_timestamp(struct sk_buff *skb)
{
struct phy_device *phydev;
struct sk_buff *clone;
unsigned int type;
if (!skb->sk)
return;
type = classify(skb);
if (type == PTP_CLASS_NONE)
return;
phydev = skb->dev->phydev;
if (likely(phydev->drv->txtstamp)) {
clone = skb_clone_sk(skb);
if (!clone)
return;
phydev->drv->txtstamp(phydev, clone, type);
}
}
The classify() call is an overhead that runs a minimal BPF dissector to
classify a network packet to
determine the PTP class. For the default non PTP case this returns
PTP_CLASS_NONE. The BPF classifier is just 3-4 BPF branches (depending on the
protocol), so it's a very small overhead per packet in the default non-PTP
cases.
I ran some perf timings on TCP data being sent and received to a host
over a 100 Mbit/s ethernet between to 8 thread Xeon servers and measured
CPU cycles, instruction and branch activity with perf. 1 GB of raw data
was transferred to/from the machines using netcat on otherwises idle
systems. Each test was run 10 times and the average, standard deviation
(population) and % standard deviation was computed.
I compared a default 4.17.0-6-generic Ubuntu Cosmic kernel against the
same kernel with CONFIG_NETWORK_PHY_TIMESTAMPING. I could not observe
any noticeable impact with the CONFIG_NETWORK_PHY_TIMESTAMPING config -
mainly because the noise in the perf measurements was larger than any
detectable difference (see the % standard deviation rates).
Since I can't easily measure the performance impact any more accurately
than instruction and branch counts, I conclude that the impact of this
config is not easily measurable and too small to be a concern.
Data in a libreoffice spread sheet is attached.
I therefore deem this config is OK to be enabled for by default for our
kernels.
** Attachment added: "libreoffice spreadsheet with test results data"
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1785816/+attachment/5173042/+files/kernel-config-CONFIG_NETWORK_PHY_TIMESTAMPING.ods
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https://bugs.launchpad.net/bugs/1785816
Title:
Consider enabling CONFIG_NETWORK_PHY_TIMESTAMPING
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