Messages in this thread | | | Date | Fri, 31 Aug 2012 12:07:33 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 01/26] rcu: New rcu_user_enter() and rcu_user_exit() APIs |
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On Thu, Aug 30, 2012 at 02:05:18PM -0700, Paul E. McKenney wrote: > From: Frederic Weisbecker <fweisbec@gmail.com> > > RCU currently insists that only idle tasks can enter RCU idle mode, which > prohibits an adaptive tickless kernel (AKA nohz cpusets), which in turn > would mean that usermode execution would always take scheduling-clock > interrupts, even when there is only one task runnable on the CPU in > question. > > This commit therefore adds rcu_user_enter() and rcu_user_exit(), which > allow non-idle tasks to enter RCU idle mode. These are quite similar > to rcu_idle_enter() and rcu_idle_exit(), respectively, except that they > omit the idle-task checks. > > [ Updated to use "user" flag rather than separate check functions. ] > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.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: Daniel Lezcano <daniel.lezcano@linaro.org> > 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: 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>
A few suggestions below: an optional microoptimization and some bugfixes. With the bugfixes, and with or without the microoptimization:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c [...] > -static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval) > +static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval, > + bool user) > { > trace_rcu_dyntick("Start", oldval, 0); > - if (!is_idle_task(current)) { > + if (!is_idle_task(current) && !user) {
Microoptimization: putting the !user check first (here and in the exit function) would allow the compiler to partially inline rcu_eqs_*_common into the two trivial wrappers and constant-fold away the test for !user.
> +void rcu_idle_enter(void) > +{ > + rcu_eqs_enter(0); > +}
s/0/false/
> +void rcu_user_enter(void) > +{ > + rcu_eqs_enter(1); > +}
s/1/true/
> -static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval) > +static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval, > + int user) > { > smp_mb__before_atomic_inc(); /* Force ordering w/previous sojourn. */ > atomic_inc(&rdtp->dynticks); > @@ -464,7 +490,7 @@ static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval) > WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)); > rcu_cleanup_after_idle(smp_processor_id()); > trace_rcu_dyntick("End", oldval, rdtp->dynticks_nesting); > - if (!is_idle_task(current)) { > + if (!is_idle_task(current) && !user) {
Same micro-optimization as the enter function.
> +void rcu_idle_exit(void) > +{ > + rcu_eqs_exit(0); > +}
s/0/false/
> +void rcu_user_exit(void) > +{ > + rcu_eqs_exit(1); > +}
s/1/true/
> @@ -539,7 +586,7 @@ void rcu_irq_enter(void) > if (oldval) > trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting); > else > - rcu_idle_exit_common(rdtp, oldval); > + rcu_eqs_exit_common(rdtp, oldval, 1);
s/1/true/, and likewise in rcu_irq_exit.
- Josh Triplett
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