netdev - Re: [PATCH net-next v2] net_sched: qdisc_alloc

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Message-Id: <20120103.130353.716810185692904362.davem@davemloft.net>
Date:	Tue, 03 Jan 2012 13:03:53 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, dave.taht@...il.com
Subject: Re: [PATCH net-next v2] net_sched: qdisc_alloc_handle() can be too
 slow

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 03 Jan 2012 11:00:11 +0100

> When trying to allocate ~32768 qdiscs using autohandle mechanism, we can
> fill the space managed by kernel (handles in [8000-FFFF]:0000 range)
> 
> But O(N^2) qdisc_alloc_handle() loops 0x10000 times instead of 0x8000
> 
> time tc add qdisc add dev eth0 parent 10:7fff pfifo limit 10
> RTNETLINK answers: Cannot allocate memory
> real    1m54.826s
> user    0m0.000s
> sys     0m0.004s
> 
> INFO: rcu_sched_state detected stall on CPU 0 (t=60000 jiffies)
> 
> Half number of loops, and add a cond_resched() call.
> We hold rtnl at this point.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> CC: Dave Taht <dave.taht@...il.com>

Applied, thanks Eric.  I always cringed when I looked at this function :)

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