Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping duringidle

[Jon Hunter]

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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