We can only get IRQ/SOFTIRQ delay in total now in Delay accounting, but
getting SOFTIRQ delay and IRQ delay separetely would help users reduce
such delays in a more convenient way.
For IRQ delay, we can tuning irq CPU affinity or using threaded-irq.
For SOFTIRQ delay, we can tuning rps/xps or using kernel threads for NAPI.
And this is an example stack a task is delayed mainly by SOFTIRQ(delay
by receiving packets when sending packets):
...
ip_rcv
__netif_receive_skb_core
__netif_receive_skb
process_backlog
net_rx_action
do_softirq
__local_bh_enable_ip
ip_finish_output2
ip_finish_output
ip_output
ip_local_out
ip_send_skb
udp_send_skb
udp_sendmsg
inet_sendmsg
sock_sendmsg
__sys_sendto
do_syscall_64
__libc_sendto
...
So this patchset tries to make SOFTIRQ delay observeable in Delay
accounting and available in taskstats.
(also update tools/accounting/getdelays.c)
Also for backward compatibility, we dont want to change the meaning of
origin IRQ/SOFTIRQ delay, instead we can get real IRQ(interrupt) delay by
the origin IRQ/SOFTIRQ delay minus SOFTIRQ delay added by this patch.
With this patch, the example above results by getdelays.c:
# ./getdelays -t 4600 -d
print delayacct stats ON
TGID 4600
CPU count real total virtual total delay total delay average
3973 10700014780 10698803222 312345815813 78.617ms
IO count delay total delay average
0 0 0.000ms
SWAP count delay total delay average
0 0 0.000ms
RECLAIM count delay total delay average
0 0 0.000ms
THRASHING count delay total delay average
0 0 0.000ms
COMPACT count delay total delay average
0 0 0.000ms
WPCOPY count delay total delay average
40 266859 0.007ms
IRQ count delay total delay average
13450 17756373906 1.320ms
SOFTIRQ count delay total delay average
13450 17639154300 1.311ms
We find out SOFTIRQ impact the delay most, then tune RPS to reduce this.
