Messages in this thread | | | Date | Tue, 4 Sep 2012 18:04:22 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 01/26] rcu: New rcu_user_enter() and rcu_user_exit() APIs |
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On Fri, Aug 31, 2012 at 12:07:33PM -0700, Josh Triplett wrote: > 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:
Good catches! Due to conflicts with later commits, I added these as a separate commit.
Thanx, Paul
> 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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