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

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Date:	Mon, 1 Jul 2013 08:33:59 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu
Subject: Re: [PATCH RFC nohz_full v2 3/7] nohz_full: Add per-CPU idle-state
 tracking

On Fri, Jun 28, 2013 at 01:10:18PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.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
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