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    SubjectRe: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu
    > > Ah so it was already 25% slower even without kmalloc? I thought
    > > that was with already. That doesn't sound good. Any idea where that slowdown
    > > comes from?
    >
    > Just longer code path, I think. It calls the generic

    I did IPI measurements quite some time ago and what I remember
    from them is that IPI latencies were in the low multiple thousands cycle
    ballpark.

    > smp_call_function_mask(), which then does a popcount on the cpu mask
    > (which it needs to do anyway), sees only one bit set, and then punts to
    > the smp_call_function_single() path.

    But that is more in the a few tens of cycles (or maybe 1-2 hundreds
    if you have a NR_CPU==4096 kernel with really large cpumask)

    Doesn't really explain a 25% slowdown I would say.

    Are you sure there isn't a new cache miss in there or something? Actually
    it must be even multiple ones to account for such a slow down.

    -Andi

    --
    ak@linux.intel.com


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