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    SubjectRe: Question about usage of RCU in the input layer
    On Fri, Mar 20, 2009 at 06:50:58AM -0700, Arjan van de Ven wrote:
    > On Thu, 19 Mar 2009 21:45:41 -0700
    > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
    >
    > > > single CPU is soooo last decade ;-)
    > > > But seriously I no longer have systems that aren't dual core or SMT
    > > > in some form...
    > >
    > > OK, I will ask the stupid question...
    > >
    > > Why not delay bringing up the non-boot CPUs until later in boot?
    >
    > that'd be throwing out the baby with the bathwater... I'm trying to use
    > the other cpus to do some of the boot work (so that the total goes
    > faster); not using the other cpus would be counter productive to that.
    > (As is just sitting in synchronize_rcu() when the other cpu is
    > working.. hence this discussion ;-)

    OK, so you are definitely running multiple CPUs when the offending
    synchronize_rcu() executes, then?

    If so, here are some follow-on questions:

    1. How many synchronize_rcu() calls are you seeing on the
    critical boot path and what value of HZ are you running?

    If each synchronize_rcu() is taking (say) tens of jiffies, then,
    as Peter Zijlstra notes earlier in this thread, we need to focus
    on what is taking too long to get through its RCU read-side
    critical sections. Otherwise, if each synchronize_rcu() is
    in the 3-5 jiffy range, I may finally be forced to create an
    expedited version of the synchronize_rcu() API.

    2. If expediting is required, then the code calling synchronize_rcu()
    might or might not have any idea whether or not expediting is
    appropriate. If it does not, then we would need some sort of way
    to tell synchronize_rcu() that it should act more aggressively,
    perhaps /proc flag or kernel global variable indicating that
    boot is in progress.

    No, we do not want to make synchronize_rcu() aggressive all the
    time, as this would harm performance and energy efficiency in
    the normal runtime situation.

    So, if it turns out that synchronize_rcu()'s caller does not
    know whether or not expediting is appropriate, can the boot path
    manipulate such a flag or variable?

    3. Which RCU implementation are you using? CONFIG_CLASSIC_RCU,
    CONFIG_TREE_RCU, or CONFIG_PREEMPT_RCU?

    Thanx, Paul


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