linux-kernel - Re: [PATCH tip/core/rcu 06/26] rcu: Ignore userspace extended quiescent state by default

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Message-ID: <20120831234654.GD11771@jtriplet-mobl1>
Date:	Fri, 31 Aug 2012 16:46:54 -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, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH tip/core/rcu 06/26] rcu: Ignore userspace extended
 quiescent state by default

On Thu, Aug 30, 2012 at 02:05:23PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@...il.com>
> 
> By default we don't want to enter into RCU extended quiescent
> state while in userspace because doing this produces some overhead
> (eg: use of syscall slowpath). Set it off by default and ready to
> run when some feature like adaptive tickless need it.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Alessio Igor Bogani <abogani@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Avi Kivity <avi@...hat.com>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Geoff Levand <geoff@...radead.org>
> Cc: Gilad Ben Yossef <gilad@...yossef.com>
> Cc: Hakan Akkan <hakanakkan@...il.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Kevin Hilman <khilman@...com>
> Cc: Max Krasnyansky <maxk@...lcomm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/rcutree.c |    5 ++++-
>  kernel/rcutree.h |    1 +
>  2 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 8bbc7fb..e2fd370 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -210,6 +210,9 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
>  DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
>  	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
>  	.dynticks = ATOMIC_INIT(1),
> +#ifdef CONFIG_RCU_USER_QS
> +	.ignore_user_qs = true,
> +#endif
>  };
>  
>  static int blimit = 10;		/* Maximum callbacks per rcu_do_batch. */
> @@ -443,7 +446,7 @@ void rcu_user_enter(void)
>  
>  	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
> -	if (!rdtp->in_user) {
> +	if (!rdtp->ignore_user_qs && !rdtp->in_user) {
>  		rdtp->in_user = true;
>  		rcu_eqs_enter(1);
>  	}
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 0dd5fd6..c190582 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -103,6 +103,7 @@ struct rcu_dynticks {
>  	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
>  #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
>  #ifdef CONFIG_RCU_USER_QS
> +	bool ignore_user_qs;	    /* Treat userspace as extended QS or not */
>  	bool in_user;		    /* Is the CPU in userland from RCU POV? */
>  #endif
>  };
> -- 
> 1.7.8
> 
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