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Re: route cache DoS testing and softirqs

To: dipankar@xxxxxxxxxx
Subject: Re: route cache DoS testing and softirqs
From: Robert Olsson <Robert.Olsson@xxxxxxxxxxx>
Date: Wed, 31 Mar 2004 20:46:09 +0200
Cc: Andrea Arcangeli <andrea@xxxxxxx>, "David S. Miller" <davem@xxxxxxxxxx>, kuznet@xxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx, Robert.Olsson@xxxxxxxxxxx, paulmck@xxxxxxxxxx, akpm@xxxxxxxx
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Dipankar Sarma writes:
 > And I am not. I am still on 2.6.0 and there seems to be no NAPI support
 > for the e100 there. Should I try 2.6.4 where e100 has NAPI support ?
 > Anyway, even without softirqs on the back of hardirqs, there are
 > other ways of softirq overload as seen in Robert's setup.

Well I see some hardirq's most from TX and timer interrups and HZ=1000 can
can change the way softirq's are run a bit.

I hacked do_softirq() sources so we can understand how things work a bit 
better.  (Use fastroute stats from /proc/net/softnet_stat). 

ksoftird == softirq's sourced from ksofttirq
irqexit  == softirq's sourced from interrupt exit
bh_enbl  == softirq's sourced local_bh_enable

Before run

total    droppped tsquz    throttl  bh_enbl  ksoftird irqexit  other   
00000000 00000000 00000000 00000000 000000e8 0000017e 00030411 00000000
00000000 00000000 00000000 00000000 000000ae 00000277 00030349 00000000

After DoS (See description from previous mail)

total    droppped tsquz    throttl  bh_enbl  ksoftird irqexit  other    
00164c55 00000000 000021de 00000000 000000fc 0000229f 0003443c 00000000
001695e7 00000000 0000224d 00000000 00000162 0000236f 000342f7 00000000

So the major part of softirq's are run from irqexit and therefor out of 
scheduler control. This even with RX polling (eth0, eth2) We still have 
some TX interrupts plus timer interrupts now at 1000Hz. Which probably 
reduces the number of softirq's that ksoftirqd runs.

          CPU0       CPU1       
  0:     297156          0    IO-APIC-edge  timer
  1:        431          0    IO-APIC-edge  i8042
  2:          0          0          XT-PIC  cascade
  8:          0          0    IO-APIC-edge  rtc
 14:      34527          0    IO-APIC-edge  ide0
 26:        131          0   IO-APIC-level  eth0
 27:      22910          0   IO-APIC-level  eth1
 28:          8        124   IO-APIC-level  eth2
 29:          9      23197   IO-APIC-level  eth3
NMI:          0          0 
LOC:     297060     297059 

Attachment: softirq_monitor.pat
Description: Binary data

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