linux-kernel - Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used

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Date:	Sat, 14 Jun 2014 15:05:50 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually
 used

On Fri, Jun 13, 2014 at 10:06:06PM -0700, Paul E. McKenney wrote:
> One complication...  So if the grace period has gone on for a long time,
> and you are returning to kernel mode, RCU will need the scheduling-clock
> tick.  However, in that very same situation, if you are returning to
> idle or to NO_HZ_FULL userspace execution, RCU does -not- need the
> scheduling-clock tick set.

Right.

> One way I could do this is to have rcu_needs_cpu() return three values:
> Zero for RCU doesn't need a scheduling-clock tick for any reason,
> one if RCU needs a scheduling-clock tick only if returning to kernel
> mode, and two if RCU unconditionally needs the scheduling-clock tick.
> Would that work, or is there a better approach?

For an interrupt, based on the context tracking state, I can check where we
return afterward if we are in an interrupt using context_tracking_in_user().

Now probably rcu_needs_cpu() should check that by itself and, depending
on where we return, only tell if we keep the tick or not.

What do you think?
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