linux-kernel - Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle

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Date:	Thu, 25 Jun 2009 14:10:49 -0500
From:	Jon Hunter <jon-hunter@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	john stultz <johnstul@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during
 idle


Jon Hunter wrote:
> Jon Hunter wrote:
>> Jon Hunter wrote:
>>> +		 * Calculate the time delta for the next timer event.
>>> +		 * If the time delta exceeds the maximum time delta
>>> +		 * permitted by the current clocksource then adjust
>>> +		 * the time delta accordingly to ensure the
>>> +		 * clocksource does not wrap.
>>> +		 */
>>> +		time_delta = tick_period.tv64 * delta_jiffies;
>> Thinking about this more, although it is very unlikely, for 64-bit 
>> machines there is a chance that the above multiply could overflow if 
>> delta_jiffies is very large.
>>
>> tick_period.tv64 should always be less than NSEC_PER_SEC and so you 
>> would need delta_jiffies to be greater than 2^32 to cause overflow. On a 
>> 32-bit machine an unsigned long will not be greater than 2^32 as it is 
>> only 32-bits but this would be possible on a 64-bit machines.
>>
>> So to be safe we should make sure that delta_jiffies is not greater than 
>>   NEXT_TIMER_MAX_DELTA (2^30 - 1) before doing the multiply. If you 
>> think that this is a valid concern, then I can re-work and re-post. 
>> Sorry for not catching this before.
> 
> With regard to the above, to ensure that there are no overflows with the 
> above calculation, I re-worked this patch a little. The below should be 
> equivalent to the current code, just re-organised a little. Let me know 
> if this would be acceptable or not.

Hi Thomas, John,

Did you guys have chance to review this? Let me know if you have any 
further comments/feedback.

Cheers
Jon
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