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SubjectRe: [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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