Subject: [PATCH] rcu: increment quiescent state counter in ksoftirqd()

From: Eric Dumazet <[email-masked]>
Date: Fri, 27 Feb 2009 17:08:04 +0100

Eric Dumazet a écrit :
> Eric Dumazet a écrit :
>> Stephen Hemminger a écrit :
>>> The reader/writer lock in ip_tables is acquired in the critical path of
>>> processing packets and is one of the reasons just loading iptables can cause
>>> a 20% performance loss. The rwlock serves two functions:
>>>
>>> 1) it prevents changes to table state (xt_replace) while table is in use.
>>>    This is now handled by doing rcu on the xt_table. When table is
>>>    replaced, the new table(s) are put in and the old one table(s) are freed
>>>    after RCU period.
>>>
>>> 2) it provides synchronization when accesing the counter values.
>>>    This is now handled by swapping in new table_info entries for each cpu
>>>    then summing the old values, and putting the result back onto one
>>>    cpu.  On a busy system it may cause sampling to occur at different
>>>    times on each cpu, but no packet/byte counts are lost in the process.
>>>
>>> Signed-off-by: Stephen Hemminger <[email-masked]>
>>
>> Acked-by: Eric Dumazet <[email-masked]>
>>
>> Sucessfully tested on my dual quad core machine too, but iptables only (no ipv6 here)
>>
>> BTW, my new "tbench 8" result is 2450 MB/s, (it was 2150 MB/s not so long ago)
>>
>> Thanks Stephen, thats very cool stuff, yet another rwlock out of kernel :)
>>
> 
> While testing multicast flooding stuff, I found that "iptables -nvL" can 
> have a *very* slow response time on my dual quad core machine...
> 
> 
> # time iptables -nvL
> Chain INPUT (policy ACCEPT 416M packets, 64G bytes)
>  pkts bytes target     prot opt in     out     source               destination
> 
> Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
>  pkts bytes target     prot opt in     out     source               destination
> 
> Chain OUTPUT (policy ACCEPT 401M packets, 62G bytes)
>  pkts bytes target     prot opt in     out     source               destination
> 
> real    0m1.810s  <<<< HERE >>>>
> user    0m0.000s
> sys     0m0.001s
> 
> 
> CONFIG_NO_HZ=y
> CONFIG_HZ_1000=y
> CONFIG_HZ=1000
> 
> One cpu is 100% handling softirqs, could it be the problem ?
> 
> Cpu0  :  1.0%us, 14.7%sy,  0.0%ni, 83.3%id,  0.0%wa,  0.0%hi,  1.0%si,  0.0%st
> Cpu1  :  3.6%us, 23.2%sy,  0.0%ni, 71.6%id,  0.0%wa,  0.0%hi,  1.7%si,  0.0%st
> Cpu2  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,100.0%si,  0.0%st
> Cpu3  :  2.7%us, 23.9%sy,  0.0%ni, 71.1%id,  0.7%wa,  0.0%hi,  1.7%si,  0.0%st
> Cpu4  :  1.3%us, 14.3%sy,  0.0%ni, 83.3%id,  0.0%wa,  0.0%hi,  1.0%si,  0.0%st
> Cpu5  :  1.0%us, 14.2%sy,  0.0%ni, 83.4%id,  0.0%wa,  0.0%hi,  1.3%si,  0.0%st
> Cpu6  :  0.3%us,  7.0%sy,  0.0%ni, 92.4%id,  0.0%wa,  0.0%hi,  0.3%si,  0.0%st
> Cpu7  :  0.7%us,  8.0%sy,  0.0%ni, 90.0%id,  0.7%wa,  0.0%hi,  0.7%si,  0.0%st

Hi Paul

I found following patch helps if one cpu is looping inside ksoftirqd()

synchronize_rcu() now completes in 40 ms instead of 1800 ms.

Thank you

[PATCH] rcu: increment quiescent state counter in ksoftirqd()

If a machine is flooded by network frames, a cpu can loop 100% of its time
inside ksoftirqd() without calling schedule().
This can delay RCU grace period to insane values. 

Adding rcu_qsctr_inc() call in ksoftirqd() solves this problem.

Signed-off-by: Eric Dumazet <[email-masked]>
---
diff --git a/kernel/softirq.c b/kernel/softirq.c
index bdbe9de..9041ea7 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -626,6 +626,7 @@ static int ksoftirqd(void * __bind_cpu)
 			preempt_enable_no_resched();
 			cond_resched();
 			preempt_disable();
+			rcu_qsctr_inc((long)__bind_cpu);
 		}
 		preempt_enable();
 		set_current_state(TASK_INTERRUPTIBLE);

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