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Re: route cache DoS testing and softirqs

To: Andrea Arcangeli <andrea@xxxxxxx>
Subject: Re: route cache DoS testing and softirqs
From: Dipankar Sarma <dipankar@xxxxxxxxxx>
Date: Tue, 30 Mar 2004 20:13:24 +0530
Cc: linux-kernel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx, Robert Olsson <Robert.Olsson@xxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxx>, Dave Miller <davem@xxxxxxxxxx>, Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxx>
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On Tue, Mar 30, 2004 at 12:29:26AM +0200, Andrea Arcangeli wrote:
> the only real starvation you can claim is in presence of an _hard_irq
> flood, not a softirq one. Ingo had some patch for the hardirq
> throttling, unfortunately those pathes were mixed with irrelevant
> softirq changes, but the hardirq part of these patches was certainly
> valid (though in most business environments I imagine if one is under
> hardirq attack in the local ethernet, the last worry is probably the
> throttling of hardirqs ;)

Hmm.. What about firewalls and routers on the internet ? Shouldn't
they care ?

> So you're simply asking the ksoftirqd offloading to become more
> aggressive, and to make the softirq even more scheduler friendly,
> something I never had a reason to do yet, since ksoftirqd already
> eliminates the starvation issue, and secondly because I did care about
> the performance of softirq first (delaying softirqs is derimental for
> performance if it happens frequently w/o this kind of flood-load). I
> even got a patch for 2.4 doing this kind of changes to the softirqd for
> similar reasons on embedded systems where the cpu spent on the softirqs
> would been way too much under attack. I had to back it out since it was
> causing drop of performance in specweb or something like that and nobody
> but the embdedded people needed it.  But now here we've a case where it
> makes even more sense since the hardirq aren't strictly related to this
> load, this load with the rcu-routing-cache is just about letting the
> scheduler go together witn an intensive softirq load. So we can try
> again with a truly userspace throttling of the softirqs (and in 2.4 I
> didn't change the nice from 19 to -20 so maybe this will just work
> perfectly).

Tried it and it didn't work. I still got dst cache overflows. I will dig
out more numbers about what what happened - is ksoftirqd a pig still or
we are mostly doing short softirq bursts on the back of a hardirq
flood.

Thanks
Dipankar

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