Messages in this thread | | | Date | Mon, 16 Jan 2017 16:32:27 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 10/20] rcu: Add functions to test for trivial grace periods |
| |
On Mon, Jan 16, 2017 at 12:01:29AM -0800, Josh Triplett wrote: > On Sat, Jan 14, 2017 at 01:13:11AM -0800, Paul E. McKenney wrote: > > Under some circumstances, RCU grace periods are zero cost. For > > RCU-preempt, this is the case during boot, and for RCU-bh and RCU-sched, > > this is the case if there is only one CPU. This means that RCU users > > might wish to dispense with grace-period-avoidance strategies when > > grace periods are zero cost, so this commit adds rcu_trivial_gp(), > > rcu_bh_trivial_gp(), and rcu_sched_trivial_gp() to test for these > > conditions. Because the conditions leading to zero-cost grace periods > > can change at any time (for example, when a second CPU is onlined), these > > functions should be used as performance hints, and must not be relied > > on for correctness. For example, even if rcu_trivial_gp() returns true, > > you are required to invoke synchronize_rcu(). > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Do you have anything planned that uses these functions? > > Exposing this information, rather than just letting callers call > synchronize_rcu() and sometimes get "free" grace periods, seems > potentially error-prone. > > If you keep these functions, please expand the comments above them to > explicitly include the explanation in the commit message, and > specifically that you must always call the corresponding synchronize > function even if these functions return true.
Hmmm...
Someone asked for this, I failed to add their Reported-by, and I do not remember who it was. I have tagged this commit with trivial_gp.2017.01.17a and am dropping it from my commits for the 4.11 merge window. If someone speaks up for it, I can always add it in later.
The use case was something about doing batching if !rcu_trivial_gp(), but not bothering if rcu_trivial_gp(), which means that grace periods are free anyway. So performance, no risk of incorrectness. If used correctly, anyway...
Thanx, Paul
> > include/linux/rcupdate.h | 8 ++++++++ > > include/linux/rcutiny.h | 10 ++++++++++ > > include/linux/rcutree.h | 2 ++ > > kernel/rcu/tree.c | 24 ++++++++++++++++++++++++ > > kernel/rcu/tree_plugin.h | 11 +++++++++++ > > 5 files changed, 55 insertions(+) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 01f71e1d2e94..a6222478b87d 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -293,6 +293,7 @@ void __rcu_read_lock(void); > > void __rcu_read_unlock(void); > > void rcu_read_unlock_special(struct task_struct *t); > > void synchronize_rcu(void); > > +bool rcu_trivial_gp(void); > > > > /* > > * Defined as a macro as it is a very low level header included from > > @@ -448,6 +449,13 @@ bool __rcu_is_watching(void); > > #define RCU_SCHEDULER_INIT 1 > > #define RCU_SCHEDULER_RUNNING 2 > > > > +#ifndef CONFIG_PREEMPT_RCU > > +static inline bool rcu_trivial_gp(void) > > +{ > > + return rcu_sched_trivial_gp(); > > +} > > +#endif /* #ifndef CONFIG_PREEMPT_RCU */ > > + > > /* > > * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic > > * initialization and destruction of rcu_head on the stack. rcu_head structures > > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h > > index ac81e4063b40..a77dafe79813 100644 > > --- a/include/linux/rcutiny.h > > +++ b/include/linux/rcutiny.h > > @@ -82,6 +82,16 @@ static inline void synchronize_sched_expedited(void) > > synchronize_sched(); > > } > > > > +static inline bool rcu_sched_trivial_gp(void) > > +{ > > + return true; > > +} > > + > > +static inline bool rcu_bh_trivial_gp(void) > > +{ > > + return true; > > +} > > + > > static inline void kfree_call_rcu(struct rcu_head *head, > > rcu_callback_t func) > > { > > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h > > index 63a4e4cf40a5..fcd61cb08851 100644 > > --- a/include/linux/rcutree.h > > +++ b/include/linux/rcutree.h > > @@ -47,6 +47,8 @@ static inline void rcu_virt_note_context_switch(int cpu) > > void synchronize_rcu_bh(void); > > void synchronize_sched_expedited(void); > > void synchronize_rcu_expedited(void); > > +bool rcu_sched_trivial_gp(void); > > +bool rcu_bh_trivial_gp(void); > > > > void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func); > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index d7b63b88434b..ed5a17aca281 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -3293,6 +3293,18 @@ void synchronize_sched(void) > > EXPORT_SYMBOL_GPL(synchronize_sched); > > > > /** > > + * rcu_sched_trivial_gp - Are RCU-sched grace periods trivially zero cost? > > + * > > + * Returns true if RCU-sched grace periods are currently zero cost, which > > + * they are if there is only one CPU. Note that unless you take steps to > > + * prevent it, the number of CPUs might change at any time. > > + */ > > +bool rcu_sched_trivial_gp(void) > > +{ > > + return rcu_blocking_is_gp(); > > +} > > + > > +/** > > * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed. > > * > > * Control will return to the caller some time after a full rcu_bh grace > > @@ -3320,6 +3332,18 @@ void synchronize_rcu_bh(void) > > EXPORT_SYMBOL_GPL(synchronize_rcu_bh); > > > > /** > > + * rcu_bh_trivial_gp - Are RCU-bh grace periods trivially zero cost? > > + * > > + * Returns true if RCU-bh grace periods are currently zero cost, which > > + * they are if there is only one CPU. Note that unless you take steps to > > + * prevent it, the number of CPUs might change at any time. > > + */ > > +bool rcu_bh_trivial_gp(void) > > +{ > > + return rcu_blocking_is_gp(); > > +} > > + > > +/** > > * get_state_synchronize_rcu - Snapshot current RCU state > > * > > * Returns a cookie that is used by a later call to cond_synchronize_rcu() > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 56583e764ebf..e92d67a3fad2 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -680,6 +680,17 @@ void synchronize_rcu(void) > > EXPORT_SYMBOL_GPL(synchronize_rcu); > > > > /** > > + * rcu_trivial_gp - Are RCU grace periods trivially zero cost? > > + * > > + * Returns true if RCU grace periods are currently zero cost, which > > + * they are during boot. > > + */ > > +bool rcu_trivial_gp(void) > > +{ > > + return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE; > > +} > > + > > +/** > > * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete. > > * > > * Note that this primitive does not necessarily wait for an RCU grace period > > -- > > 2.5.2 > > >
| |