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

[Ingo Molnar <mingo@elte.hu>]

* 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