From: Ashutosh Kaushik <k.ashut...@samsung.com>
The patch fixes the issues with check of global flag "rps_needed" in RX Path
(which process packets in TCP/IP stack like netif_rx and netif_receive_skb
functions)
These functions have flag CONFIG RPS which is enabled default in kernel and to
enter in RPS mode, it depends on variable rps_needed.
This variable is updated whenever value in
/sys/class/net/<device>/queues/rx-0/rps_cpus is being changed.
There are 2 scenarios where it is executing extra piece of code even when
results would be same every time:-
1) Suppose in system more than one networking devices are connected i.e. wired
(eth0) and wireless (wlan0).
If I enable RPS using above method for wlan0, then it sets atomic variable
in rps_needed flag which is global.
Now,whenever traffic uses wired network, then it will execute that extra
piece of code in RX path because of only dependency on global rps_needed
variable as below:
#ifdef CONFIG_RPS
if (static_key_false(&rps_needed)) {
struct rps_dev_flow voidflow, *rflow = &voidflow;
int cpu = get_rps_cpu(skb->dev, skb, &rflow);
if (cpu >= 0) {
ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
rcu_read_unlock();
return ret;
}
}
#endif
while every time, value returned from get_rps_cpu will be < 0 as
rps_map value is not set for this network device using
/sys/class/net/device/queues/rx-0/rps_cpus.
And it will every time execute get_rps_cpu function with fail case in
IF condition which will be an extra overhead for packet processing.
2) Another scenario is as: Suppose that we have enable RPS for wireless device
say wlan0 using above specified method which will set value of rps_needed.
After doing test with set value of rps_cpus in sysfs, we do rmmod driver of
wireless device.
Next time again when we do insmod without rebooting system, it always hit
below code with fail case:
#ifdef CONFIG_RPS
if (static_key_false(&rps_needed)) {
struct rps_dev_flow voidflow, *rflow = &voidflow;
int cpu;
preempt_disable();
rcu_read_lock();
cpu = get_rps_cpu(skb->dev, skb, &rflow);
if (cpu < 0)
cpu = smp_processor_id();
ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
rcu_read_unlock();
preempt_enable();
} else
#endif
The reason behind this overhead of hitting this code with false case is same
because before doing rmmod, we enabled RPS which set rps_needed flag.
Next time when we do insmod, it will just create entry for network device in
sysfs with default value which is 0 for rps_cpus. It implies that RPS is
disable for that device.
But due to unchanged value of rps_needed variable, it goes into IF condition
every time (which is failed in get_rps_cpu function) even rps_cpus is 0.
Because if we do not enable RPS for that network device, rps_map is not set.
The patch adds a check to these two RX functions which will check RPS
availability locally for device specific.
Signed-off-by: Ashutosh Kaushik <k.ashut...@samsung.com>
---
net/core/dev.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/net/core/dev.c b/net/core/dev.c
index 5df6cbc..1aa4402 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3531,12 +3531,14 @@ drop:
static int netif_rx_internal(struct sk_buff *skb)
{
int ret;
+ struct netdev_rx_queue *dev_rxqueue = skb->dev->_rx;
net_timestamp_check(netdev_tstamp_prequeue, skb);
trace_netif_rx(skb);
#ifdef CONFIG_RPS
- if (static_key_false(&rps_needed)) {
+ if (static_key_false(&rps_needed) &&
+ dev_rxqueue->rps_map && dev_rxqueue->rps_map->len) {
struct rps_dev_flow voidflow, *rflow = &voidflow;
int cpu;
@@ -3986,6 +3988,7 @@ static int __netif_receive_skb(struct sk_buff *skb)
static int netif_receive_skb_internal(struct sk_buff *skb)
{
int ret;
+ struct netdev_rx_queue *dev_rxqueue = skb->dev->_rx;
net_timestamp_check(netdev_tstamp_prequeue, skb);
@@ -3995,7 +3998,8 @@ static int netif_receive_skb_internal(struct sk_buff *skb)
rcu_read_lock();
#ifdef CONFIG_RPS
- if (static_key_false(&rps_needed)) {
+ if (static_key_false(&rps_needed) &&
+ dev_rxqueue->rps_map && dev_rxqueue->rps_map->len) {
struct rps_dev_flow voidflow, *rflow = &voidflow;
int cpu = get_rps_cpu(skb->dev, skb, &rflow);
--
1.9.1
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