On Tue, Jan 08, 2013 at 03:08:26AM +0100, Frederic Weisbecker wrote:
> If we are interrupting userspace, we don't need to keep
> the tick for RCU: quiescent states don't need to be reported
> because we soon run in userspace and local callbacks are handled
> by the nocb threads.
>
> CHECKME: Do the nocb threads actually handle the global
> grace period completion for local callbacks?
First answering this for the nocb stuff in mainline: In this case,
the grace-period startup is handled by the CPU that is not a nocb
CPU, and there has to be at least one. The grace-period completion
is handled by the grace-period kthreads. The nocbs CPU need do
nothing, at least assuming that it gets back into dyntick-idle
(or adaptive tickless) state reasonably quickly.
Second for the version in -rcu: In this case, the nocb kthreads
register the need for a grace period using a new mechanism that
pushes the need up the rcu_node tree. The grace-period completion
is again handled by the grace-period kthreads. This allows all
CPUs to be nocbs CPUs.
So, in either case, yes, the below code should be safe as long as
the CPU gets into an RCU-idle state quickly (as in within a few
milliseconds or so).
Thanx, Paul
> Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
> Cc: Alessio Igor Bogani <abog...@kernel.org>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Chris Metcalf <cmetc...@tilera.com>
> Cc: Christoph Lameter <c...@linux.com>
> Cc: Geoff Levand <ge...@infradead.org>
> Cc: Gilad Ben Yossef <gi...@benyossef.com>
> Cc: Hakan Akkan <hakanak...@gmail.com>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Li Zhong <zh...@linux.vnet.ibm.com>
> Cc: Namhyung Kim <namhyung....@lge.com>
> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: Paul Gortmaker <paul.gortma...@windriver.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> ---
> kernel/time/tick-sched.c | 6 +++---
> 1 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 1cd93a9..ecba8b7 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -22,6 +22,7 @@
> #include <linux/module.h>
> #include <linux/irq_work.h>
> #include <linux/posix-timers.h>
> +#include <linux/context_tracking.h>
>
> #include <asm/irq_regs.h>
>
> @@ -604,10 +605,9 @@ static bool can_stop_full_tick(int cpu)
>
> /*
> * Keep the tick if we are asked to report a quiescent state.
> - * This must be further optimized (avoid checks for local callbacks,
> - * ignore RCU in userspace, etc...
> + * This must be further optimized (avoid checks for local callbacks)
> */
> - if (rcu_pending(cpu)) {
> + if (!context_tracking_in_user() && rcu_pending(cpu)) {
> trace_printk("Can't stop: RCU pending\n");
> return false;
> }
> --
> 1.7.5.4
>
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