On Thu, Mar 23, 2006 at 11:35:50PM -0800, Matt Helsley ([email protected]) wrote:
> I would argue preemption should be disabled around the if-block at the
> very least. Suppose your rate limit is 10k calls/sec and you have 4
> procs. Each proc has a sequence of three instructions:
>
> load fsevent_sum into register rx (rx <= 1000)
> rx++ (rx <= 1001)
> store contents of register rx in fsevent_sum (fsevent_sum <= 1001)
>
>
> Now consider the following sequence of steps:
>
> load fsevent_sum into rx (rx <= 1000)
> <preempted>
> <3 other processors each manage to increment the sum by 3333 bringing us
> to 9999>
> <resumed>
> rx++ (rx <= 1001)
> store contents of rx in fsevent_sum (fsevent_sum <= 1001)
>
> So every processor now thinks it won't exceed the rate limit by
> generating more events when in fact we've just exceeded the limit. So,
> unless my example is flawed, I think you need to disable preemption
> here.
Doesn't it just exceed the limit by one event per cpu?
> Also, even if you simply disable preemption couldn't this cause the
> cache line containing the sum to bounce frequently on large SMP systems?
>
> <snip>
>
> Cheers,
> -Matt Helsley
--
Evgeniy Polyakov
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