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

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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!

Ingo


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