Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()

[Boqun Feng]

On Tue, Dec 20, 2016 at 01:59:14PM +0800, Boqun Feng wrote:
> On Mon, Dec 19, 2016 at 09:09:13PM -0800, Paul E. McKenney wrote:
> > On Mon, Dec 19, 2016 at 11:15:15PM +0800, Boqun Feng wrote:
> > > On Thu, Dec 15, 2016 at 02:51:36PM +0000, Colin Ian King wrote:
> > > > On 15/12/16 14:42, Boqun Feng wrote:
> > > > > On Thu, Dec 15, 2016 at 12:04:59PM +0000, Mark Rutland wrote:
> > > > >> On Thu, Dec 15, 2016 at 10:42:03AM +0800, Boqun Feng wrote:
> > > > >>> ->qsmask of an RCU leaf node is usually more sparse than the
> > > > >>> corresponding cpu_possible_mask. So replace the
> > > > >>> for_each_leaf_node_possible_cpu() in force_qs_rnp() with
> > > > >>> for_each_leaf_node_cpu() to save several checks.
> > > > >>>
> > > > >>> [Note we need to use "1UL << bit" instead of "1 << bit" to generate the
> > > > >>> corresponding mask for a bit because @mask is unsigned long, this was
> > > > >>> spotted by Colin Ian King <colin.king@xxxxxxxxxxxxx> and CoverityScan in
> > > > >>> a previous version of this patch.]
> > > > >>
> > > > >> Nit: This note can go now that we use leaf_node_cpu_bit(). ;)
> > > > >>
> > > > > 
> > > > > I kinda keep this here for honoring the effort of finding out this bug
> > > > > from Colin, but yes, it's no longer needed here for the current code.
> > > > 
> > > > Yep, remove it.
> > > > 
> > > 
> > > Paul, here is a modified version of this patch, what I only did is
> > > removing this note.
> > > 
> > > Besides I rebased the whole series on the current rcu/dev branch of -rcu
> > > tree, on this very commit:
> > > 
> > > 	8e9b2521b18a ("doc: Quick-Quiz answers are now inline")
> > > 
> > > And I put the latest version at
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git leaf-node
> > > 
> > > If you thought it's better, I could send a v3 ;-)
> > 
> > I would feel better about this patchset if it reduced the number of lines
> > of code rather than increasing them.  That said, part of the increase
> > is a commment.  Still, I am not convinced that the extra level of macro
> > is carrying its weight.
> > 
> > dbf18a2422e2 ("rcu: Introduce for_each_leaf_node_cpu()")
> > 
> > 	The commit log needs a bit of wordsmithing.
> > 
> > 	The added WARN_ON_ONCE(!cpu_possible(cpu)) still seems strange.
> > 	What is its purpose, really?  What does its triggering tell you?
> > 	What other checks did you consider as an alternative?
> > 
> 
> The check is an over-case one, it's introduced because I'm worried about
> some code outside the RCU code mis-sets the ->qsmask* or ->expmask* on
> an "impossible" CPU. I will explanation later in more details.
> 
> > 	And if you are going to add checks of this type, should you
> > 	also check for this being a leaf rcu_node structure?
> > 
> 
> I don't think I want to check that, and I don't think check
> cpu_possible(cpu) in the macro is similar to that.
> 
> > 3f0b4ba1fe94 ("rcu: Use for_each_leaf_node_cpu() in RCU stall checking")
> > 
> > 	This does look a bit nicer, but why the added blank lines?
> > 	Are they really helping?
> > 
> > 	The commit log seems a bit misplaced.  This code is almost never
> > 	executed (once per 21 seconds at the most), so performance really
> > 	isn't a consideration.	The simpler-looking code might be.
> > 
> > fd799f1ac7b7 ("rcu: Use for_each_leaf_node_cpu() in ->expmask iteration")
> > 
> > 	Ditto on blank lines.
> > 
> > 	Again, this code is executed per expedited grace period, so
> > 	performance really isn't a big deal.  More of a big deal than
> > 	the stall-warning code, but we still are way off of any fastpath.
> > 
> > 69a1baedbf42 ("rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()")
> > 
> > 	Ditto again on blank lines.
> > 
> > 	And on the commit log.  This code is executed about once
> > 	per several jiffies, and on larger machines, per 20 jiffies
> > 	or so.  Performance really isn't a consideration.
> > 
> > 7b00e50e3efb ("rcu: Use for_each_leaf_node_cpu() in online CPU iteration")
> > 
> > 	And another ditto on blank lines.
> > 
> > 	This code executes once per CPU-hotplug operation, so again isn't
> > 	at all performance critical.
> > 
> > In short, if you are trying to sell this to me as a significant performance
> > boost, I am not buying.  The added WARN_ON_ONCE() looks quite dubious,
> 
> Yep, it won't help the performance a lot, but it 
> 
> 1)	helps the performance in theory, because it iterates less CPUs
> 
> 2)	makes code cleaner. By "cleaner", I mean we can a) affort more
> 	blank lines to make loops separated from other code and b)
> 	descrease the indent levels for those loops. But, yes I should
> 	add those points in the commit log, because those are more
> 	visible effects.
> 
> > though perhaps I am misunderstanding its purpose.  My assumption is
> > that you want to detect missing UL suffixes on bitmask constants, in
> > which case I bet there is a better way.
> > 
> 
> The WARN_ON_ONCE() is not for detecting missing UL suffixes on bitmask
> constatns, and we don't need to check that, because we use
> leaf_node_cpu_id() now. As I said, this is an over-case check, and we
> can drop if we guarante that CPUs masked in ->qsmask* and ->expmask*
> must be a "possible" CPU, IOW, ->qsmask* and ->expmask* are the subsets
> (with offset fixed by ->grplo) of cpu_possible_mask.
> 
> Hmm.. and I just check the code, the initial values of ->qsmask* and
> ->expmask* are from ->qsmaskinitnext and ->expmaskinitnext, and the
> latter two are set in rcu_cpu_starting() since commit
> 
> 	7ec99de36f40 ("rcu: Provide exact CPU-online tracking for RCU")
> 
> , and rcu_cpu_starting() only set the corresponding bit of _this_ cpu in
> a leaf node's ->qsmaskinitnext and ->expmaskinitnext. So looks like we
> are safe to remove the WARN_ON_ONCE() check, because a ever-running CPU
> must be a possible CPU, IIRC.
> 
> But this brings a side question, is the callsite of rcu_cpu_starting()
> is correct? Given rcu_cpu_starting() ignores the @cpu parameter and only

By "callsite", I mean we call rcu_cpu_starting() in a
for_each_online_cpu() loop. And that doesn't seem making sense to me,
because rcu_cpu_starting() doesn't use its parameter @cpu. So I made the
following untested patch to fix this.

Thoughts?

> set _this_ cpu's bit in a leaf node?
> 

Regards,
Boqun

-------------------------------->8
From: Boqun Feng <boqun.feng@xxxxxxxxx>
Date: Tue, 20 Dec 2016 15:10:57 +0800
Subject: [PATCH] rcu: Rename rcu_cpu_starting() to rcu_this_cpu_starting()

rcu_cpu_starting() was introduced at commit:

	7ec99de36f40 ("rcu: Provide exact CPU-online tracking for RCU")

, and was to inform RCU core the onlining of _this_ cpu, and it was
implemented as its purpose, which made the parameter @cpu useless.

It's better if we remove the unnecessary parameter and rename it to
rcu_this_cpu_starting(), which fits its functionality well. Besides, in
rcu_init(), we actually loop over all online CPUs but keep notifying
that the boot cpu is online to RCU core, so we'd better pull the
notification part out of the loop.

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/rcupdate.h |  2 +-
 kernel/cpu.c             |  2 +-
 kernel/rcu/tree.c        | 17 ++++++++---------
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 813074714a95..f23c9dafbda9 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -335,7 +335,7 @@ void rcu_sched_qs(void);
 void rcu_bh_qs(void);
 void rcu_check_callbacks(int user);
 void rcu_report_dead(unsigned int cpu);
-void rcu_cpu_starting(unsigned int cpu);
+void rcu_this_cpu_starting(void);
 
 #ifndef CONFIG_TINY_RCU
 void rcu_end_inkernel_boot(void);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5df20d6d1520..63778ed6b598 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -966,7 +966,7 @@ void notify_cpu_starting(unsigned int cpu)
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	enum cpuhp_state target = min((int)st->target, CPUHP_AP_ONLINE);
 
-	rcu_cpu_starting(cpu);	/* Enables RCU usage on this CPU. */
+	rcu_this_cpu_starting();	/* Enables RCU usage on this CPU. */
 	while (st->state < target) {
 		st->state++;
 		cpuhp_invoke_callback(cpu, st->state, true, NULL);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b9d3c0e30935..c5862aef7e21 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4002,13 +4002,13 @@ int rcutree_dead_cpu(unsigned int cpu)
 }
 
 /*
- * Mark the specified CPU as being online so that subsequent grace periods
- * (both expedited and normal) will wait on it.  Note that this means that
- * incoming CPUs are not allowed to use RCU read-side critical sections
- * until this function is called.  Failing to observe this restriction
- * will result in lockdep splats.
+ * Mark this CPU(CPU that is currently running this function) as being online
+ * so that subsequent grace periods (both expedited and normal) will wait on
+ * it.  Note that this means that incoming CPUs are not allowed to use RCU
+ * read-side critical sections until this function is called.  Failing to
+ * observe this restriction will result in lockdep splats.
  */
-void rcu_cpu_starting(unsigned int cpu)
+void rcu_this_cpu_starting(void)
 {
 	unsigned long flags;
 	unsigned long mask;
@@ -4376,10 +4376,9 @@ void __init rcu_init(void)
 	 * or the scheduler are operational.
 	 */
 	pm_notifier(rcu_pm_notify, 0);
-	for_each_online_cpu(cpu) {
+	for_each_online_cpu(cpu)
 		rcutree_prepare_cpu(cpu);
-		rcu_cpu_starting(cpu);
-	}
+	rcu_this_cpu_starting(); /* Start RCU on the booting CPU */
 }
 
 #include "tree_exp.h"
-- 
2.10.2