On Thu, Apr 01, 2004 at 02:07:50AM +0530, Dipankar Sarma wrote:

On Wed, Mar 31, 2004 at 08:46:09PM +0200, Robert Olsson wrote:
Content-Description: message body text

quoted

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.

So, NAPI or not we get userland stalls due to packetflooding.

indeed, the most of the softirq load happens within irqs even with NAPI
as we were talking about, so Alexey and DaveM were wrong about the
hardirq load being non significant.

Maybe the problem is simply that NAPI should be tuned more aggressively,
it may have to poll for a longer time before giving up.

Looking at some of the old patches we discussed privately, it seems
this is what was done earlier -

1. Use rcu-softirq.patch which provides call_rcu_bh() for softirqs
   only.

this is the one I prefer if it performs.

2. Limit non-ksoftirqd softirqs - get a measure of userland stall (using
   an api rcu_grace_period(cpu)) and if it is too long, expire
   the timeslice of the current process  and start sending everything to 
   ksoftirqd.

yep, this may be desiderable eventually just to be fair with tasks, but
I believe it's partly an orthogonal with the rcu grace period length.

By reducing the softirq time at the back of a hardirq or local_bh_enable(),
we should be able to bring a bit more fairness. I am working on the
patches, will test and publish later. 

I consider this as the approch number 2 too.