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net: napi: add hard irqs deferral feature

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Back in commit 3b47d30396 ("net: gro: add a per device gro flush timer")
we added the ability to arm one high resolution timer, that we used
to keep not-complete packets in GRO engine a bit longer, hoping that further
frames might be added to them.

Since then, we added the napi_complete_done() interface, and commit
364b605573 ("net: busy-poll: return busypolling status to drivers")
allowed drivers to avoid re-arming NIC interrupts if we made a promise
that their NAPI poll() handler would be called in the near future.

This infrastructure can be leveraged, thanks to a new device parameter,
which allows to arm the napi hrtimer, instead of re-arming the device
hard IRQ.

We have noticed that on some servers with 32 RX queues or more, the chit-chat
between the NIC and the host caused by IRQ delivery and re-arming could hurt
throughput by ~20% on 100Gbit NIC.

In contrast, hrtimers are using local (percpu) resources and might have lower
cost.

The new tunable, named napi_defer_hard_irqs, is placed in the same hierarchy
than gro_flush_timeout (/sys/class/net/ethX/)

By default, both gro_flush_timeout and napi_defer_hard_irqs are zero.

This patch does not change the prior behavior of gro_flush_timeout
if used alone : NIC hard irqs should be rearmed as before.

One concrete usage can be :

echo 20000 >/sys/class/net/eth1/gro_flush_timeout
echo 10 >/sys/class/net/eth1/napi_defer_hard_irqs

If at least one packet is retired, then we will reset napi counter
to 10 (napi_defer_hard_irqs), ensuring at least 10 periodic scans
of the queue.

On busy queues, this should avoid NIC hard IRQ, while before this patch IRQ
avoidance was only possible if napi->poll() was exhausting its budget
and not call napi_complete_done().

This feature also can be used to work around some non-optimal NIC irq
coalescing strategies.

Having the ability to insert XX usec delays between each napi->poll()
can increase cache efficiency, since we increase batch sizes.

It also keeps serving cpus not idle too long, reducing tail latencies.

Co-developed-by: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
2020-04-22 09:13:27 -07:00
committed by David S. Miller
parent e6acd2b6e8
commit 6f8b12d661
3 changed files with 38 additions and 11 deletions
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18
net/core/net-sysfs.c
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@@ -382,6 +382,23 @@ static ssize_t gro_flush_timeout_store(struct device *dev,
}
NETDEVICE_SHOW_RW(gro_flush_timeout, fmt_ulong);
static int change_napi_defer_hard_irqs(struct net_device *dev, unsigned long val)
{
dev->napi_defer_hard_irqs = val;
return 0;
}
static ssize_t napi_defer_hard_irqs_store(struct device *dev,
struct device_attribute *attr,
const char *buf, size_t len)
{
if (!capable(CAP_NET_ADMIN))
return -EPERM;
return netdev_store(dev, attr, buf, len, change_napi_defer_hard_irqs);
}
NETDEVICE_SHOW_RW(napi_defer_hard_irqs, fmt_dec);
static ssize_t ifalias_store(struct device *dev, struct device_attribute *attr,
const char *buf, size_t len)
{
@@ -545,6 +562,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
&dev_attr_flags.attr,
&dev_attr_tx_queue_len.attr,
&dev_attr_gro_flush_timeout.attr,
&dev_attr_napi_defer_hard_irqs.attr,
&dev_attr_phys_port_id.attr,
&dev_attr_phys_port_name.attr,
&dev_attr_phys_switch_id.attr,