Messages in this thread | | | Date | Fri, 24 Mar 2006 22:04:13 +0800 | From | yang.y.yi@gmail ... | Subject | Re: Connector: Filesystem Events Connector |
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On 3/24/06, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote: > On Fri, Mar 24, 2006 at 01:42:17AM -0800, Matt Helsley (matthltc@us.ibm.com) > wrote: > > On Fri, 2006-03-24 at 11:11 +0300, Evgeniy Polyakov wrote: > > > 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? > > > > The example exceeds it by one at the time of the final store. Thanks to > > the fact that the value is then 1001 it may shortly be exceeded by much > > more than 1. > > + > + if (jiffies - last <= fsevent_ratelimit) { > + if (fsevent_sum > fsevent_burst_limit) > + return -2; > + fsevent_sum++; > > Only process (and not process' syscall) can preempt us here, > so fsevent_sum can only exceed fsevent_burst_limit by one per process > (process can not preempt itself, so when it has finished syscall which > ends up in event generation, fsevent_sum will be increased). > > + } else { > + last = jiffies; > + fsevent_sum = 0; > + } > > Actually, since jiffies and atomic operations are already used, I do not > think addition of new atomic_inc_return or something similar will > even somehow change the picture. rate limit is just for some abnormal cases, for example, an application leads to a unlimited event loop, it should be very few, in the worst case, the user application which induced this case will terminate, this should be a reasonable result. rate limit is intent to prevent this kind of case and avoid DoS of system. > > > > 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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