From: Josh Triplett <josh@joshtriplett.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
laijs@cn.fujitsu.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
fweisbec@gmail.com, patches@linaro.org
Subject: Re: [PATCH RFC tip/core/rcu 17/41] rcu: Remove single-rcu_node optimization in rcu_start_gp()
Date: Wed, 1 Feb 2012 18:13:14 -0800 [thread overview]
Message-ID: <20120202021314.GM29058@leaf> (raw)
In-Reply-To: <1328125319-5205-17-git-send-email-paulmck@linux.vnet.ibm.com>
On Wed, Feb 01, 2012 at 11:41:35AM -0800, Paul E. McKenney wrote:
> The grace-period initialization sequence in rcu_start_gp() has a special
> case for systems where the rcu_node tree is a single rcu_node structure.
> This made sense some years ago when systems were smaller and up to 64
> CPUs could share a single rcu_node structure, but now that large systems
> are common and a given leaf rcu_node structure can support only 16 CPUs
> (due to lock contention on the rcu_node's ->lock field), this optimization
> is almost never taken. And even the small mobile platforms that might
> make use of it might rather have the kernel text reduction.
>
> Therefore, this commit removes the check for single-rcu_node trees.
This optimization would continue to work on laptops for a while longer.
:)
That said, I do agree that reducing code size and complexity seems
preferable. If someone wants an optimization like this, they'd probably
do better to compile RCU with a low compile-time limit on the number of
CPUs, which would at least theoretically allow the compiler to get
similar results through optimization. (I don't know if that works in
practice with the current code structure and the current intelligence of
GCC.)
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
next prev parent reply other threads:[~2012-02-02 2:13 UTC|newest]
Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-01 19:41 [PATCH RFC 0/41] RCU commits for 3.4 Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 01/41] rcu: Bring RTFP.txt up to date Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 02/41] rcu: Improve synchronize_rcu() diagnostics Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock Paul E. McKenney
2012-02-02 0:55 ` Josh Triplett
2012-02-02 16:20 ` Paul E. McKenney
2012-02-02 19:56 ` Josh Triplett
2012-02-02 20:42 ` Paul E. McKenney
2012-02-03 9:04 ` Josh Triplett
2012-02-03 18:05 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic for misaligned rcu_head structures Paul E. McKenney
2012-02-02 1:00 ` Josh Triplett
2012-02-02 16:22 ` Paul E. McKenney
2012-02-02 20:11 ` Josh Triplett
2012-02-02 1:01 ` Josh Triplett
2012-02-02 16:27 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 05/41] rcu: Avoid waking up CPUs having only kfree_rcu() callbacks Paul E. McKenney
2012-02-02 1:15 ` Josh Triplett
2012-02-02 16:34 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 06/41] rcu: Move RCU_TRACE to lib/Kconfig.debug Paul E. McKenney
2012-02-02 1:39 ` Josh Triplett
2012-02-02 17:05 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 07/41] s390: Convert call_rcu() to kfree_rcu() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 08/41] tcm_fc: " Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 09/41] ipv4: " Paul E. McKenney
2012-02-01 19:49 ` David Miller
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 10/41] " Paul E. McKenney
2012-02-01 19:50 ` David Miller
2012-02-02 0:24 ` Josh Triplett
2012-02-02 15:56 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 11/41] mac80211: " Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 12/41] rcu: Simplify offline processing Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 13/41] rcu: Make rcutorture flag online/offline failures Paul E. McKenney
2012-02-02 1:46 ` Josh Triplett
2012-02-02 17:08 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 14/41] rcu: Limit lazy-callback duration Paul E. McKenney
2012-02-02 2:03 ` Josh Triplett
2012-02-02 17:13 ` Paul E. McKenney
2012-02-03 4:07 ` Josh Triplett
2012-02-03 5:54 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 15/41] rcu: Check for callback invocation from offline CPUs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 16/41] rcu: Don't make callbacks go through second full grace period Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 17/41] rcu: Remove single-rcu_node optimization in rcu_start_gp() Paul E. McKenney
2012-02-02 2:13 ` Josh Triplett [this message]
2012-02-02 17:16 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 18/41] rcu: Protect __rcu_read_unlock() against scheduler-using irq handlers Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 19/41] rcu: Streamline code produced by __rcu_read_unlock() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 20/41] rcu: Prevent RCU callbacks from executing before scheduler initialized Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 21/41] rcu: Inform RCU of irq_exit() activity Paul E. McKenney
2012-02-02 2:30 ` Josh Triplett
2012-02-02 17:30 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 22/41] rcu: Simplify unboosting checks Paul E. McKenney
2012-02-02 2:38 ` Josh Triplett
2012-02-02 17:48 ` Paul E. McKenney
2012-02-03 4:23 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 23/41] rcu: Clean up straggling rcu_preempt_needs_cpu() name Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 24/41] rcu: Check for idle-loop entry while in RCU read-side critical section Paul E. McKenney
2012-02-02 5:13 ` Josh Triplett
2012-02-02 17:50 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 25/41] rcu: Make rcu_sleep_check() also check rcu_lock_map Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 26/41] rcu: Note that rcu_access_pointer() can be used for teardown Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 27/41] rcu: Remove #ifdef CONFIG_SMP from TREE_RCU Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 28/41] rcu: Set RCU CPU stall times via sysfs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 29/41] rcu: Print scheduling-clock information on RCU CPU stall-warning messages Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 30/41] rcutorture: Permit holding off CPU-hotplug operations during boot Paul E. McKenney
2012-02-02 5:43 ` Josh Triplett
2012-02-02 17:56 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 31/41] rcu: Add CPU-stall capability to rcutorture Paul E. McKenney
2012-02-02 5:53 ` Josh Triplett
2012-02-02 9:15 ` Julia Lawall
2012-02-02 18:03 ` Paul E. McKenney
2012-02-02 18:00 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 32/41] rcu: Update stall-warning documentation Paul E. McKenney
2012-02-02 5:56 ` Josh Triplett
2012-02-02 18:18 ` Paul E. McKenney
2012-02-03 5:42 ` Josh Triplett
2012-02-03 5:58 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 33/41] rcu: Make boolean rcutorture parameters be of type "bool" Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 34/41] rcu: Check for illegal use of RCU from offlined CPUs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 35/41] rcu: Move synchronize_sched_expedited() to rcutree.c Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 36/41] rcu: No interrupt disabling for rcu_prepare_for_idle() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 37/41] lockdep: Add CPU-idle/offline warning to lockdep-RCU splat Paul E. McKenney
2012-02-02 6:07 ` Josh Triplett
2012-02-02 18:30 ` Paul E. McKenney
2012-02-03 6:12 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs Paul E. McKenney
2012-02-02 6:11 ` Josh Triplett
2012-02-02 18:31 ` Paul E. McKenney
2012-02-03 9:17 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy before declaring a CPU to be offline Paul E. McKenney
2012-02-02 6:12 ` Josh Triplett
2012-02-02 18:27 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 40/41] rcu: Call out dangers of expedited RCU primitives Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 41/41] rcu: Trace only after NULL-pointer check Paul E. McKenney
2012-02-02 0:18 ` [PATCH RFC tip/core/rcu 01/41] rcu: Bring RTFP.txt up to date Josh Triplett
2012-02-02 1:33 ` Paul E. McKenney
2012-02-02 2:01 ` Josh Triplett
2012-02-02 16:47 ` Paul E. McKenney
2012-02-02 22:32 ` Josh Triplett
2012-02-03 18:00 ` Paul E. McKenney
2012-02-02 22:47 ` [PATCH RFC 0/41] RCU commits for 3.4 Kevin Hilman
2012-02-02 23:58 ` Paul E. McKenney
2012-02-03 19:54 ` Kevin Hilman
2012-02-06 7:04 ` Paul E. McKenney
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