From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 10/20] rcu: Add functions to test for trivial grace periods
Date: Mon, 16 Jan 2017 16:32:27 -0800 [thread overview]
Message-ID: <20170117003227.GV5238@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170116080129.3svwfwyjkdnhcjcm@x>
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
> >
>
next prev parent reply other threads:[~2017-01-17 3:05 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-14 9:12 [PATCH tip/core/rcu 0/20] Miscellaneous fixes for 4.11 Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 01/20] rcu: update: Make RCU_EXPEDITE_BOOT be the default Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 02/20] lockdep: Make RCU suspicious-access splats use pr_err Paul E. McKenney
2017-01-16 7:53 ` Josh Triplett
2017-01-17 1:06 ` Paul E. McKenney
2017-01-16 17:21 ` Peter Zijlstra
2017-01-16 20:59 ` Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 03/20] Fix: Disable sys_membarrier when nohz_full is enabled Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 04/20] rcu: Only dump stalled-tasks stacks if there was a real stall Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 05/20] rcu: Remove unneeded rcu_process_callbacks() declarations Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 06/20] rcu: Remove unused but set variable Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 07/20] rcu: Remove short-term CPU kicking Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 08/20] rcu: Add long-term " Paul E. McKenney
2017-01-16 7:58 ` Josh Triplett
2017-01-17 1:07 ` Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 09/20] rcu: Once again use NMI-based stack traces in stall warnings Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 10/20] rcu: Add functions to test for trivial grace periods Paul E. McKenney
2017-01-16 8:01 ` Josh Triplett
2017-01-17 0:32 ` Paul E. McKenney [this message]
2017-01-14 9:13 ` [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state Paul E. McKenney
2017-01-16 8:08 ` Josh Triplett
2017-01-16 11:38 ` Paul E. McKenney
2017-01-16 17:11 ` Peter Zijlstra
2017-01-17 0:54 ` Paul E. McKenney
2017-01-17 10:51 ` Michal Hocko
2017-01-17 12:05 ` Paul E. McKenney
2017-01-17 12:11 ` Michal Hocko
2017-01-18 2:10 ` Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 12/20] rcu: Re-enable TASKS_RCU for User Mode Linux Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 13/20] rcu: Don't wake rcuc/X kthreads on NOCB CPUs Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 14/20] rcu: Add comment headers to expedited-grace-period counter functions Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 15/20] rcu: Make rcu_cpu_starting() use its "cpu" argument Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 16/20] rcu: Enable RCU tracepoints by default to aid in debugging Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 17/20] rcu: Fix comment in rcu_organize_nocb_kthreads() Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 18/20] llist: Clarify comments about when locking is needed Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 19/20] rcu: Eliminate unused expedited_normal counter Paul E. McKenney
2017-01-14 9:13 ` [PATCH tip/core/rcu 20/20] rcu: Add lockdep checks to synchronous expedited primitives Paul E. McKenney
2017-01-16 8:09 ` [PATCH tip/core/rcu 0/20] Miscellaneous fixes for 4.11 Josh Triplett
2017-01-17 1:07 ` Paul E. McKenney
2017-01-18 2:53 ` Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 01/18] rcu: update: Make RCU_EXPEDITE_BOOT be the default Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 02/18] lockdep: Make RCU suspicious-access splats use pr_err Paul E. McKenney
2017-01-21 20:40 ` Josh Triplett
2017-01-23 19:30 ` Paul E. McKenney
2017-01-23 22:33 ` Josh Triplett
2017-01-24 0:13 ` Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 03/18] Fix: Disable sys_membarrier when nohz_full is enabled Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 04/18] rcu: Only dump stalled-tasks stacks if there was a real stall Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 05/18] rcu: Remove unneeded rcu_process_callbacks() declarations Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 06/18] rcu: Remove unused but set variable Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 07/18] rcu: Add long-term CPU kicking Paul E. McKenney
2017-01-21 20:41 ` Josh Triplett
2017-01-21 20:42 ` Josh Triplett
2017-01-23 19:34 ` Paul E. McKenney
2017-01-23 20:25 ` Josh Triplett
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 08/18] rcu: Remove short-term " Paul E. McKenney
2017-01-21 20:43 ` Josh Triplett
2017-01-23 19:36 ` Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 09/18] rcu: Once again use NMI-based stack traces in stall warnings Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 10/18] rcu: Re-enable TASKS_RCU for User Mode Linux Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 11/18] rcu: Don't wake rcuc/X kthreads on NOCB CPUs Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 12/18] rcu: Add comment headers to expedited-grace-period counter functions Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 13/18] rcu: Make rcu_cpu_starting() use its "cpu" argument Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 14/18] rcu: Enable RCU tracepoints by default to aid in debugging Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 15/18] rcu: Fix comment in rcu_organize_nocb_kthreads() Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 16/18] llist: Clarify comments about when locking is needed Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 17/18] rcu: Eliminate unused expedited_normal counter Paul E. McKenney
2017-01-18 2:53 ` [PATCH v2 tip/core/rcu 18/18] rcu: Add lockdep checks to synchronous expedited primitives Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 0/18] Miscellaneous fixes for 4.11 Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 01/18] rcu: update: Make RCU_EXPEDITE_BOOT be the default Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 02/18] lockdep: Make RCU suspicious-access splats use pr_err Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 03/18] Fix: Disable sys_membarrier when nohz_full is enabled Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 04/18] rcu: Only dump stalled-tasks stacks if there was a real stall Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 05/18] rcu: Remove unneeded rcu_process_callbacks() declarations Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 06/18] rcu: Remove unused but set variable Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 07/18] rcu: Add long-term CPU kicking Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 08/18] rcu: Remove short-term " Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 09/18] rcu: Once again use NMI-based stack traces in stall warnings Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 10/18] rcu: Re-enable TASKS_RCU for User Mode Linux Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 11/18] rcu: Don't wake rcuc/X kthreads on NOCB CPUs Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 12/18] rcu: Add comment headers to expedited-grace-period counter functions Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 13/18] rcu: Make rcu_cpu_starting() use its "cpu" argument Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 14/18] rcu: Enable RCU tracepoints by default to aid in debugging Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 15/18] rcu: Fix comment in rcu_organize_nocb_kthreads() Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 16/18] llist: Clarify comments about when locking is needed Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 17/18] rcu: Eliminate unused expedited_normal counter Paul E. McKenney
2017-01-24 21:51 ` [PATCH v3 tip/core/rcu 18/18] rcu: Add lockdep checks to synchronous expedited primitives Paul E. McKenney
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