netdev - Re: [PATCH net-next] time: jiffies_delta_to_clock

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Date:	Mon, 13 Aug 2012 17:52:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Maciej Żenczykowski <maze@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	hank <pyu@...hat.com>
Subject: Re: [PATCH net-next] time: jiffies_delta_to_clock_t() helper to the
 rescue

On Thu, 9 Aug 2012, Eric Dumazet wrote:

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Various /proc/net files sometimes report crazy timer values, expressed
> in clock_t units.
> 
> This happens when an expired timer delta (expires - jiffies) is passed
> to jiffies_to_clock_t().
>
> This function has an overflow in :
> 
> return div_u64((u64)x * TICK_NSEC, NSEC_PER_SEC / USER_HZ);
> 
> commit cbbc719fccdb8cb (time: Change jiffies_to_clock_t() argument type
> to unsigned long) only got around the problem.
> 
> As we cant output negative values in /proc/net/tcp without breaking
> various tools, I suggest adding a jiffies_delta_to_clock_t() wrapper
> that caps the negative delta to a 0 value.

That's correct for 64bit, but for 32bit you do the same for valid
timer values where expires < jiffies. So you might output 0 for valid
armed timers for quite a while around the point where jiffies are
close to the wrapping point.

Dunno if that's an issue, but it want's to be documented at least.

Thanks,

	tglx
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