Messages in this thread | | | Date | Mon, 7 Jan 2013 20:06:26 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 26/33] rcu: Don't keep the tick for RCU while in userspace |
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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 <fweisbec@gmail.com> > Cc: Alessio Igor Bogani <abogani@kernel.org> > Cc: Andrew Morton <akpm@linux-foundation.org> > 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: Ingo Molnar <mingo@kernel.org> > Cc: Li Zhong <zhong@linux.vnet.ibm.com> > Cc: Namhyung Kim <namhyung.kim@lge.com> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Paul Gortmaker <paul.gortmaker@windriver.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Thomas Gleixner <tglx@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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