linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
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
> 

  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20151006164445.GA9600@cloud \
    --to=josh@joshtriplett.org \
    --cc=akpm@linux-foundation.org \
    --cc=bobby.prani@gmail.com \
    --cc=boqun.feng@gmail.com \
    --cc=dhowells@redhat.com \
    --cc=dipankar@in.ibm.com \
    --cc=dvhart@linux.intel.com \
    --cc=edumazet@google.com \
    --cc=fweisbec@gmail.com \
    --cc=jiangshanlai@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=mingo@kernel.org \
    --cc=oleg@redhat.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).