Re: [PATCH RFC nohz_full v2 3/7] nohz_full: Add per-CPU idle-state tracking

[Josh Triplett <josh@joshtriplett.org>]

On Fri, Jun 28, 2013 at 01:10:18PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds the code that updates the rcu_dyntick structure's
> new fields to track the per-CPU idle state based on interrupts and
> transitions into and out of the idle loop (NMIs are ignored because NMI
> handlers cannot cleanly read out the time anyway).  This code is similar
> to the code that maintains RCU's idea of per-CPU idleness, but differs
> in that RCU treats CPUs running in user mode as idle, where this new
> code does not.
[...]
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -2380,6 +2380,76 @@ static void rcu_kick_nohz_cpu(int cpu)
>  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
>  
>  /*
> + * Invoked to note exit from irq or task transition to idle.  Note that
> + * usermode execution does -not- count as idle here!  The caller must
> + * have disabled interrupts.

Can you explain in the comments why this code doesn't treat userspace as
idle/quiesced?

- Josh Triplett