netdev - Re: [PATCH] NET : rt_check_expire() can take a long time, add a cond

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Message-Id: <20071115.005720.12599943.davem@davemloft.net>
Date:	Thu, 15 Nov 2007 00:57:20 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	andi@...stfloor.org, netdev@...r.kernel.org
Subject: Re: [PATCH] NET : rt_check_expire() can take a long time, add a
 cond_resched()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Thu, 15 Nov 2007 09:25:59 +0100

> Please note that :
> 
> if (need_resched())
>      cond_resched();
> 
> will re-test need_resched() once cond_resched() is called.
> 
> So it may sound unnecessary but in the rt_check_expire() case, with a loop 
> potentially doing XXX.XXX iterations, being able to bypass the function call 
> is a clear win (in my bench case, 25 ms instead of 88 ms). Impact on I-cache 
> is irrelevant here as this rt_check_expires() runs once every 60 sec.

BTW, Eric, initially I was going to recommend that you do
something like:

	if ((goal % SOME_POWER_OF_2) == 0)
		cond_resched();

to mitigate this cost but decided it wasn't worth the bother.
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