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    SubjectRe: [2.6.16 PATCH] Connector: Filesystem Events Connector
    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.


    > Cheers,
    > -Matt Helsley

    --
    Evgeniy Polyakov
    -
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