netdev - High process latencies due to MPC5200 FEC hard- soft-irq processing

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Message-ID: <4C3C6A24.3000906@grandegger.com>
Date:	Tue, 13 Jul 2010 15:29:08 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Netdev@...r.kernel.org
CC:	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: High process latencies due to MPC5200 FEC hard- soft-irq processing

Hello,

we realized, that multiple ping floods (ping -f) can cause very large
high-priority process latencies (up to a many seconds) on a MPC5200
PowerPC system with FEC NAPI support. The latencies are measured with

  # cyclictest -p 80 -n

The problem is that processing of the ICMP pakets in the Hard-Irq and
Soft-IRQ context can last for a long time without returning to the
scheduler. Reducing MAX_SOFTIRQ_RESTART from 10 to 2 helps - the latency
goes down to 35 ms with 2 "ping -f" - but it's not a configurable
parameter, even if it somehow depends on the CPU power. And using the
-rt patches seems overkill to me. Any other ideas or comments on how to
get rid of such high process latencies?

Wolfgang.
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