netdev - Re: [RFT 2/4] Add mod_timer

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Date:	Wed, 18 Feb 2009 11:07:58 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Rick Jones <rick.jones2@...com>,
	Eric Dumazet <dada1@...mosbay.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, tglx@...utronix.de,
	Martin Josefsson <gandalf@...g.westbo.se>
Subject: Re: [RFT 2/4] Add mod_timer_noact

Ingo Molnar wrote:
>> -extern int __mod_timer(struct timer_list *timer, unsigned long expires);
>> +extern int __mod_timer(struct timer_list *timer, unsigned long expires, int activate);
> 
> This is not really acceptable, it slows down every single 
> add_timer() and mod_timer() call in the kernel with a flag that 
> has one specific value in all but your case. There's more than 
> 2000 such callsites in the kernel.
> 
> Why dont you use something like this instead:
> 
> 	if (del_timer(timer))
> 		add_timer(timer);

We need to avoid having a timer that was deleted by one CPU
getting re-added by another, but want to avoid taking the
conntrack lock for every timer update. The timer-internal
locking is enough for this as long as we have a mod_timer
variant that forwards a timer, but doesn't activate it in
case it isn't active already.
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