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    SubjectRe: [PATCH] v4 Teach RCU that idle task is not quiscent state at boot

    * Paul E. McKenney <> wrote:

    > OK, alternatives...
    > o Reverse the roles of the idle and init threads during startup,
    > so that there is initially no idle thread.
    > However, there appears to be a fair amount of code that assumes
    > that there is always an idle thread.
    > o As above, but create both the init and idle threads early so
    > that there always is an idle thread that happens not to be
    > running during boot.
    > This would work, but seems to me to be uglier than the flag.
    > o Stop using idle_cpu() in rcu_check_callbacks(), instead keeping
    > a per-CPU "cpu_is_idle" variable that is set upon entry to the
    > various idle() loops and cleared upon exit. It would be OK to
    > take interrupts while "cpu_is_idle" is set.
    > The disadvantage here is that there are quite a few idle loops,
    > and it would be necessary to change them all. Missing one or
    > two could result in indefinite grace periods on the affected
    > systems.
    > o Drop idle as a quiescent state, as is already the case for
    > rcupreempt.
    > This would result in indefinite grace-period delays for
    > rcuclassic, but would actually work for rcutree. Except that
    > it would cause rcutree to IPI each and every idle CPU for
    > every grace period if !CONFIG_NO_HZ. I expect that this
    > overhead would far exceed that of the extra flag check in
    > rcu_check_callbacks().
    > So I still like the flag check. Any alternatives that I am missing?

    Indeed none of the alternatives looks particularly appealing, so
    i concur. Thanks Paul for analyzing it so thoroughly!


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