Messages in this thread | | | Date | Fri, 24 Mar 2006 11:11:49 +0300 | From | Evgeniy Polyakov <> | Subject | Re: [2.6.16 PATCH] Connector: Filesystem Events Connector |
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On Thu, Mar 23, 2006 at 11:35:50PM -0800, Matt Helsley (matthltc@us.ibm.com) 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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