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

Josh Triplett
2012-08-31T23:47:06Z

On Thu, Aug 30, 2012 at 02:05:23PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@gmail.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@gmail.com>
> Cc: Alessio Igor Bogani <abogani@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Geoff Levand <geoff@infradead.org>
> Cc: Gilad Ben Yossef <gilad@benyossef.com>
> Cc: Hakan Akkan <hakanakkan@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Kevin Hilman <khilman@ti.com>
> Cc: Max Krasnyansky <maxk@qualcomm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.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
>