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    SubjectRe: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it
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    On Wed, Mar 21, 2012 at 4:54 PM, Christoph Lameter <cl@linux.com> wrote:
    > On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
    >
    >> If RCU is waiting for the current CPU to complete a grace
    >> period, don't turn off the tick. Unlike dynctik-idle, we
    >> are not necessarily going to enter into rcu extended quiescent
    >> state, so we may need to keep the tick to note current CPU's
    >> quiescent states.
    >
    > Is there any way for userspace to know that the tick is not off yet due to
    > this? It would make sense for us to have busy loop in user space that
    > waits until the OS has completed all processing if that avoids future
    > latencies for the application.
    >

    I previously suggested having the user register to receive a signal
    when the tick
    is turned off. Since the tick is always turned off the user task is
    the current task
    by design, *I think* you can simply mark the signal pending when you
    turn the tick off.

    The user would register a signal handler to set a flag when it is
    called and then busy
    loop waiting for a flag to clear.

    Checking this approach is on my todo list, but alas I had no progress
    with it yet :-)

    Gilad



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