You need to know which IRQ is 1e100000.ethernet, for example, on my mi router 3g:
root@LEDE:~# cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
8: 13375254 13375255 13375247 13375239 MIPS GIC Local 1 timer
9: 3494997 0 0 0 MIPS GIC 63 IPI call
10: 0 25030334 0 0 MIPS GIC 64 IPI call
11: 0 0 24823552 0 MIPS GIC 65 IPI call
12: 0 0 0 27268437 MIPS GIC 66 IPI call
13: 745167 0 0 0 MIPS GIC 67 IPI resched
14: 0 893177 0 0 MIPS GIC 68 IPI resched
15: 0 0 885124 0 MIPS GIC 69 IPI resched
16: 0 0 0 863919 MIPS GIC 70 IPI resched
18: 12 0 0 0 MIPS GIC 33 serial
19: 0 0 0 0 MIPS GIC 29 xhci-hcd:usb1
20: 186597845 1998 75988 0 MIPS GIC 10 1e100000.ethernet
21: 2 0 0 0 MIPS GIC 30 gsw
22: 24272789 0 0 0 MIPS GIC 11 mt7603e
23: 14764792 0 0 0 MIPS GIC 31 mt76x2e
1e100000.ethernet is on IRQ 20, so I typed this:
echo 4 > /proc/irq/20/smp_affinity
And it worked.