netdev - Re: [PATCH] IPV4 : convert rt_check_expire() from softirq processing to workqueue

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Date:	Sat, 15 Sep 2007 10:58:08 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] IPV4 : convert rt_check_expire() from softirq
 processing to workqueue

From: Eric Dumazet <dada1@...mosbay.com>
Date: Wed, 12 Sep 2007 15:34:44 +0200

> On loaded/big hosts, rt_check_expire() if of litle use, because it
> generally breaks out of its main loop because of a jiffies change.
> 
> It can take a long time (read : timer invocations) to actually
> scan the whole hash table, freeing unused entries.
> 
> Converting it to use a workqueue instead of softirq is a nice
> move because we can allow rt_check_expire() to do the scan
> it is supposed to do, without hogging the CPU.
> 
> This has an impact on the average number of entries in cache, 
> reducing ram usage. Cache is more responsive to parameter
> changes (/proc/sys/net/ipv4/route/gc_timeout and
> /proc/sys/net/ipv4/route/gc_interval)
> 
> Note: Maybe the default value of gc_interval (60 seconds)
> is too high, since this means we actually need 5 (300/60)
> invocations to scan the whole table.
> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

Applied, thanks Eric.  I'm rebasing my net-2.6.24 tree over the
weekend so it may take a little time for this to show up.

It just occurred to me that we need to fix something in these two
workqueue conversions.  If it runs for more than a jiffy this can
unfairly penalize other tasks that want to run on that cpu.

Therefore, we need to add a need_resched() or similar check in the
loop.
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