Re: [PATCH 16/41] rcu: Restart the tick on non-responding adaptivenohz CPUs

[Paul E. McKenney]

On Tue, May 01, 2012 at 01:54:50AM +0200, Frederic Weisbecker wrote:
> When a CPU in adaptive nohz mode doesn't respond to complete
> a grace period, issue it a specific IPI so that it restarts
> the tick and chases a quiescent state.

Hello, Frederic,

I don't understand the need for this patch.  If the CPU is in
adaptive-tick mode, RCU should see it as being in dyntick-idle mode,
right?  If so, shouldn't RCU have already recognized the CPU as being
in an extended quiescent state?

Or is this a belt-and-suspenders situation?

							Thanx, Paul

> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Alessio Igor Bogani <abogani@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Avi Kivity <avi@xxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: Geoff Levand <geoff@xxxxxxxxxxxxx>
> Cc: Gilad Ben Yossef <gilad@xxxxxxxxxxxxx>
> Cc: Hakan Akkan <hakanakkan@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Kevin Hilman <khilman@xxxxxx>
> Cc: Max Krasnyansky <maxk@xxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Stephen Hemminger <shemminger@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  kernel/rcutree.c |   17 +++++++++++++++++
>  1 files changed, 17 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e141c7e..3fffc26 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -50,6 +50,7 @@
>  #include <linux/wait.h>
>  #include <linux/kthread.h>
>  #include <linux/prefetch.h>
> +#include <linux/cpuset.h>
> 
>  #include "rcutree.h"
>  #include <trace/events/rcu.h>
> @@ -302,6 +303,20 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
> 
>  #ifdef CONFIG_SMP
> 
> +static void cpuset_update_rcu_cpu(int cpu)
> +{
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +
> +	if (cpuset_cpu_adaptive_nohz(cpu))
> +		smp_cpuset_update_nohz(cpu);
> +
> +	local_irq_restore(flags);
> +#endif
> +}
> +
>  /*
>   * If the specified CPU is offline, tell the caller that it is in
>   * a quiescent state.  Otherwise, whack it with a reschedule IPI.
> @@ -325,6 +340,8 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
>  		return 1;
>  	}
> 
> +	cpuset_update_rcu_cpu(rdp->cpu);
> +
>  	/*
>  	 * The CPU is online, so send it a reschedule IPI.  This forces
>  	 * it through the scheduler, and (inefficiently) also handles cases
> -- 
> 1.7.5.4
> 

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