Re: [bisected] pre-3.16 regression on open() scalability

[Dave Hansen <dave.hansen@intel.com>]

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On 06/13/2014 03:45 PM, Paul E. McKenney wrote:
>> > Could the additional RCU quiescent states be causing us to be doing more
>> > RCU frees that we were before, and getting less benefit from the lock
>> > batching that RCU normally provides?
> Quite possibly.  One way to check would be to use the debugfs files
> rcu/*/rcugp, which give a count of grace periods since boot for each
> RCU flavor.  Here "*" is rcu_preempt for CONFIG_PREEMPT and rcu_sched
> for !CONFIG_PREEMPT.

With the previously-mentioned workload, rcugp's "age" averages 9 with
the old kernel (or RCU_COND_RESCHED_LIM at a high value) and 2 with the
current kernel which contains this regression.

I also checked the rate and sources for how I'm calling cond_resched.
I'm calling it 5x for every open/close() pair in my test case, which
take about 7us.  So, _cond_resched() is, on average, only being called
every microsecond.  That doesn't seem _too_ horribly extreme.

>  3895.165846 |     8)               |  SyS_open() {
>  3895.165846 |     8)   0.065 us    |    _cond_resched();
>  3895.165847 |     8)   0.064 us    |    _cond_resched();
>  3895.165849 |     8)   2.406 us    |  }
>  3895.165849 |     8)   0.199 us    |  SyS_close();
>  3895.165850 |     8)               |  do_notify_resume() {
>  3895.165850 |     8)   0.063 us    |    _cond_resched();
>  3895.165851 |     8)   0.069 us    |    _cond_resched();
>  3895.165852 |     8)   0.060 us    |    _cond_resched();
>  3895.165852 |     8)   2.194 us    |  }
>  3895.165853 |     8)               |  SyS_open() {

The more I think about it, the more I think we can improve on a purely
call-based counter.

First, it couples the number of cond_resched() directly calls with the
benefits we see out of RCU.  We really don't *need* to see more grace
periods if we have more cond_resched() calls.

It also ends up eating a new cacheline in a bunch of pretty hot paths.
It would be nice to be able to keep the fast path part of this as at
least read-only.

Could we do something (functionally) like the attached patch?  Instead
of counting cond_resched() calls, we could just specify some future time
by which we want have a quiescent state.  We could even push the time to
be something _just_ before we would have declared a stall.


[-- Attachment #2: rcu-halfstall.patch --]
[-- Type: text/x-patch, Size: 2696 bytes --]



---

 b/arch/x86/kernel/nmi.c    |    6 +++---
 b/include/linux/rcupdate.h |    7 +++----
 b/kernel/rcu/update.c      |    4 ++--
 3 files changed, 8 insertions(+), 9 deletions(-)

diff -puN include/linux/rcupdate.h~rcu-halfstall include/linux/rcupdate.h
--- a/include/linux/rcupdate.h~rcu-halfstall	2014-06-17 14:08:19.596464173 -0700
+++ b/include/linux/rcupdate.h	2014-06-17 14:15:40.335598696 -0700
@@ -303,8 +303,8 @@ bool __rcu_is_watching(void);
  * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
  */
 
-extern u64 RCU_COND_RESCHED_LIM;	/* ms vs. 100s of ms. */
-DECLARE_PER_CPU(int, rcu_cond_resched_count);
+extern u64 RCU_COND_RESCHED_EVERY_THIS_JIFFIES;
+DECLARE_PER_CPU(unsigned long, rcu_cond_resched_at_jiffies);
 void rcu_resched(void);
 
 /*
@@ -321,8 +321,7 @@ void rcu_resched(void);
  */
 static inline bool rcu_should_resched(void)
 {
-	return raw_cpu_inc_return(rcu_cond_resched_count) >=
-	       RCU_COND_RESCHED_LIM;
+	return raw_cpu_read(rcu_cond_resched_at_jiffies) >= jiffies;
 }
 
 /*
diff -puN arch/x86/kernel/nmi.c~rcu-halfstall arch/x86/kernel/nmi.c
--- a/arch/x86/kernel/nmi.c~rcu-halfstall	2014-06-17 14:11:28.442072042 -0700
+++ b/arch/x86/kernel/nmi.c	2014-06-17 14:12:04.664723690 -0700
@@ -88,13 +88,13 @@ __setup("unknown_nmi_panic", setup_unkno
 
 static u64 nmi_longest_ns = 1 * NSEC_PER_MSEC;
 
-u64 RCU_COND_RESCHED_LIM = 256;
+u64 RCU_COND_RESCHED_EVERY_THIS_JIFFIES = 100;
 static int __init nmi_warning_debugfs(void)
 {
 	debugfs_create_u64("nmi_longest_ns", 0644,
 			arch_debugfs_dir, &nmi_longest_ns);
-	debugfs_create_u64("RCU_COND_RESCHED_LIM", 0644,
-			arch_debugfs_dir, &RCU_COND_RESCHED_LIM);
+	debugfs_create_u64("RCU_COND_RESCHED_EVERY_THIS_JIFFIES", 0644,
+			arch_debugfs_dir, &RCU_COND_RESCHED_EVERY_THIS_JIFFIES);
 	return 0;
 }
 fs_initcall(nmi_warning_debugfs);
diff -puN kernel/rcu/update.c~rcu-halfstall kernel/rcu/update.c
--- a/kernel/rcu/update.c~rcu-halfstall	2014-06-17 14:12:50.768834979 -0700
+++ b/kernel/rcu/update.c	2014-06-17 14:17:14.166894075 -0700
@@ -355,7 +355,7 @@ early_initcall(check_cpu_stall_init);
  * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
  */
 
-DEFINE_PER_CPU(int, rcu_cond_resched_count);
+DEFINE_PER_CPU(unsigned long, rcu_cond_resched_at_jiffies);
 
 /*
  * Report a set of RCU quiescent states, for use by cond_resched()
@@ -364,7 +364,7 @@ DEFINE_PER_CPU(int, rcu_cond_resched_cou
 void rcu_resched(void)
 {
 	preempt_disable();
-	__this_cpu_write(rcu_cond_resched_count, 0);
+	__this_cpu_write(rcu_cond_resched_at_jiffies, jiffies + RCU_COND_RESCHED_EVERY_THIS_JIFFIES);
 	rcu_note_context_switch(smp_processor_id());
 	preempt_enable();
 }
_