From: Josh Triplett <josh@joshtriplett.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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,
Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers
Date: Tue, 6 Oct 2015 09:44:45 -0700 [thread overview]
Message-ID: <20151006164445.GA9600@cloud> (raw)
In-Reply-To: <1444148028-11551-4-git-send-email-paulmck@linux.vnet.ibm.com>
On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote:
> From: Boqun Feng <boqun.feng@gmail.com>
>
> Because preempt_disable() maps to barrier() for non-debug builds,
> it forces the compiler to spill and reload registers. Because Tree
> RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
> barrier() instances generate needless extra code for each instance of
> rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree
> RCU and bloats Tiny RCU.
>
> This commit therefore removes the preempt_disable() and preempt_enable()
> from the non-preemptible implementations of __rcu_read_lock() and
> __rcu_read_unlock(), respectively. However, for debug purposes,
> preempt_disable() and preempt_enable() are still invoked if
> CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
> atomic sections in non-preemptible kernels.
>
> This is based on an earlier patch by Paul E. McKenney, fixing
> a bug encountered in kernels built with CONFIG_PREEMPT=n and
> CONFIG_PREEMPT_COUNT=y.
This also adds explicit barrier() calls to several internal RCU
functions, but the commit message doesn't explain those at all.
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> include/linux/rcupdate.h | 6 ++++--
> include/linux/rcutiny.h | 1 +
> kernel/rcu/tree.c | 9 +++++++++
> 3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d63bb77dab35..6c3ceceb6148 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -297,12 +297,14 @@ void synchronize_rcu(void);
>
> static inline void __rcu_read_lock(void)
> {
> - preempt_disable();
> + if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> + preempt_disable();
> }
>
> static inline void __rcu_read_unlock(void)
> {
> - preempt_enable();
> + if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> + preempt_enable();
> }
>
> static inline void synchronize_rcu(void)
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index c8a0722f77ea..4c1aaf9cce7b 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
>
> static inline void rcu_all_qs(void)
> {
> + barrier(); /* Avoid RCU read-side critical sections leaking across. */
> }
>
> #endif /* __LINUX_RCUTINY_H */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b9d9e0249e2f..93c0f23c3e45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
> */
> void rcu_note_context_switch(void)
> {
> + barrier(); /* Avoid RCU read-side critical sections leaking down. */
> trace_rcu_utilization(TPS("Start context switch"));
> rcu_sched_qs();
> rcu_preempt_note_context_switch();
> if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> rcu_momentary_dyntick_idle();
> trace_rcu_utilization(TPS("End context switch"));
> + barrier(); /* Avoid RCU read-side critical sections leaking up. */
> }
> EXPORT_SYMBOL_GPL(rcu_note_context_switch);
>
> @@ -353,12 +355,19 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
> * RCU flavors in desperate need of a quiescent state, which will normally
> * be none of them). Either way, do a lightweight quiescent state for
> * all RCU flavors.
> + *
> + * The barrier() calls are redundant in the common case when this is
> + * called externally, but just in case this is called from within this
> + * file.
> + *
> */
> void rcu_all_qs(void)
> {
> + barrier(); /* Avoid RCU read-side critical sections leaking down. */
> if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> rcu_momentary_dyntick_idle();
> this_cpu_inc(rcu_qs_ctr);
> + barrier(); /* Avoid RCU read-side critical sections leaking up. */
> }
> EXPORT_SYMBOL_GPL(rcu_all_qs);
>
> --
> 2.5.2
>
next prev parent reply other threads:[~2015-10-06 16:44 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-06 16:13 [PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.4 Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 01/13] sched: Export sched_setscheduler_nocheck Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 02/13] rcu: Use rcu_callback_t in call_rcu*() and friends Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 03/13] rcu: Use call_rcu_func_t to replace explicit type equivalents Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers Paul E. McKenney
2015-10-06 16:20 ` [Kernel networking modules.] OSI levels 2 & 3, Assistance - If anyone knows anyone in the US. North West region John D Allen, Leveridge Systems INC
2015-10-06 16:44 ` Josh Triplett [this message]
2015-10-06 17:01 ` [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers Paul E. McKenney
2015-10-06 17:16 ` Josh Triplett
2015-10-06 17:42 ` Paul E. McKenney
2015-10-06 17:46 ` Josh Triplett
2015-10-06 20:05 ` Peter Zijlstra
2015-10-06 20:18 ` Paul E. McKenney
2015-10-06 20:52 ` Peter Zijlstra
2015-10-06 21:05 ` Paul E. McKenney
2015-10-07 7:19 ` Peter Zijlstra
2015-10-06 16:13 ` [PATCH tip/core/rcu 05/13] rcu: Eliminate panic when silly boot-time fanout specified Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 06/13] rcu: Add online/offline info to stall warning message Paul E. McKenney
2015-10-06 17:15 ` Josh Triplett
2015-10-06 16:13 ` [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock() Paul E. McKenney
2015-10-06 17:18 ` Josh Triplett
2015-10-06 17:36 ` Paul E. McKenney
2015-10-06 17:43 ` Josh Triplett
2015-10-06 18:07 ` Paul E. McKenney
2015-10-06 20:07 ` Peter Zijlstra
2015-10-06 20:19 ` Paul E. McKenney
2015-10-06 20:32 ` Peter Zijlstra
2015-10-06 21:03 ` Paul E. McKenney
2015-10-07 7:20 ` Peter Zijlstra
2015-10-07 14:18 ` Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 08/13] rcu: Finish folding ->fqs_state into ->gp_state Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 09/13] rcu: Correct comment for values of ->gp_state field Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Paul E. McKenney
2015-10-06 17:21 ` Josh Triplett
2015-10-06 17:31 ` Paul E. McKenney
2015-10-06 17:36 ` Josh Triplett
2015-10-06 20:27 ` Peter Zijlstra
2015-10-06 21:02 ` Paul E. McKenney
2015-10-07 7:22 ` Peter Zijlstra
2015-10-07 14:20 ` Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless_dereference() Paul E. McKenney
2015-10-26 8:45 ` Ingo Molnar
2015-10-26 14:55 ` Paul E. McKenney
2015-10-26 18:02 ` Ingo Molnar
2015-10-27 3:37 ` Linus Torvalds
2015-10-27 5:19 ` Tejun Heo
2015-10-27 5:33 ` Paul E. McKenney
2015-10-28 8:33 ` Ingo Molnar
2015-10-28 20:35 ` Patrick Marlier
2015-10-29 0:00 ` Paul E. McKenney
2015-10-29 2:13 ` Tejun Heo
2015-10-28 20:58 ` [tip:core/rcu] fs/writeback, rcu: Don't use list_entry_rcu() for pointer offsetting in bdi_split_work_to_wbs() tip-bot for Tejun Heo
2015-10-27 5:32 ` [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless_dereference() Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 12/13] rcu: Remove deprecated rcu_lockdep_assert() Paul E. McKenney
2015-10-06 16:13 ` [PATCH tip/core/rcu 13/13] rculist: Use WRITE_ONCE() when deleting from reader-visible list Paul E. McKenney
2015-10-06 17:23 ` [PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.4 Josh Triplett
2015-10-06 17:38 ` Paul E. McKenney
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