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

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Date:	Wed, 18 Feb 2009 10:20:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.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


* Stephen Hemminger <shemminger@...tta.com> wrote:

> Introduce mod_timer_noact() which for example is to replace the calls to
> del_timer()/add_timer() in __nf_ct_refresh_acct(). It works like mod_timer()
> but doesn't activate or modify the timeout of an inactive timer which is the
> behaviour we want in order to be able to use timers as a means of
> synchronization in nf_conntrack.
> 
> A later patch will modify __nf_ct_refresh_acct() to use mod_timer_noact()
> which will then save one spin_lock_irqsave() / spin_lock_irqrestore() pair per
> conntrack timer update. This will also get rid of the race we currently have
> without adding more locking in nf_conntrack.
> 
> Signed-off-by: Martin Josefsson <gandalf@...g.westbo.se>
> 
> ---
>  include/linux/timer.h |    8 ++++++--
>  kernel/relay.c        |    2 +-
>  kernel/timer.c        |   40 +++++++++++++++++++++++++++++++++++-----
>  3 files changed, 42 insertions(+), 8 deletions(-)
> 
> --- a/include/linux/timer.h	2009-02-17 10:55:33.427785986 -0800
> +++ b/include/linux/timer.h	2009-02-17 11:04:10.291844534 -0800
> @@ -25,6 +25,9 @@ struct timer_list {
>  
>  extern struct tvec_base boot_tvec_bases;
>  
> +#define TIMER_ACT	1
> +#define TIMER_NOACT	0

Ugly flaggery.

> -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);

?

	Ingo
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