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Re: route cache DoS testing and softirqs

To: Robert Olsson <Robert.Olsson@xxxxxxxxxxx>
Subject: Re: route cache DoS testing and softirqs
From: Dipankar Sarma <dipankar@xxxxxxxxxx>
Date: Thu, 8 Apr 2004 01:18:43 +0530
Cc: Andrea Arcangeli <andrea@xxxxxxx>, "David S. Miller" <davem@xxxxxxxxxx>, kuznet@xxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx, paulmck@xxxxxxxxxx, akpm@xxxxxxxx
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On Wed, Apr 07, 2004 at 05:23:28PM +0200, Robert Olsson wrote:
> Dipankar Sarma writes:
> 
>  > Robert, you should try out rs-throttle-rcu.patch. The idea is that
>  > we don't run too many callbacks in a single rcu. In my setup,
>  > at 100kpps, I see as many as 30000 rcu callbacks in a single
>  > tasklet handler. That is likely hurting even the softirq-only
>  > RCU grace periods. Setting rcupdate.maxbatch=4 will do only 4 per
>  > tasklet thus providing more quiescent points to the system.
> 
> Hello!
> 
> No bad things happens, lots of overflows and drop in performance
> and the userland app can stall for 32 sec. We seems to spin in
> softirq to much and still don't get things done. 

Argh!! Andrea, this means that throttling rcu callbacks with
back-to-back rcu tasklets for better scheduling latency is bad
for this kind of DoS situation. I think we will have to address
the softirq limiting question.

That said, Robert, one last experiment - if you are running UP,
can you try the following patchset (should apply on top of vanilla
2.6.x) ? This implements direct invocation of callbacks instead
of waiting for rcu grace periods in UP kernel. This would be a
good data point to understand what happens.

Thanks
Dipankar

Attachment: rcu-rt-latency.patch
Description: Text document

Attachment: dcache-rcu-latency.patch
Description: Text document

Attachment: rtcache-rcu-latency.patch
Description: Text document

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