linux-kernel - Re: [PATCH] NOHZ: prevent multiplication overflow

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Message-Id: <20070529.160233.45506339.davem@davemloft.net>
Date:	Tue, 29 May 2007 16:02:33 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	tglx@...utronix.de
Cc:	michal.k.k.piotrowski@...il.com, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: [PATCH] NOHZ: prevent multiplication overflow - stop timer for
 huge timeouts

From: Thomas Gleixner <tglx@...utronix.de>
Date: Tue, 29 May 2007 23:47:39 +0200

> get_next_timer_interrupt() returns a delta of (LONG_MAX > 1) in case
> there is no timer pending. On 64 bit machines this results in a
> multiplication overflow in tick_nohz_stop_sched_tick(). 
> 
> Reported by: Dave Miller <davem@...emloft.net>
> 
> Make the return value a constant and limit the return value to a 32 bit
> value.
> 
> When the max timeout value is returned, we can safely stop the tick
> timer device. The max jiffies delta results in a 12 days timeout for
> HZ=1000.
> 
> In the long term the get_next_timer_interrupt() code needs to be
> reworked to return ktime instead of jiffies, but we have to wait until
> the last users of the original NO_IDLE_HZ code are converted.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Acked-off-by: David S. Miller <davem@...emloft.net>
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