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

[Paul E. McKenney]

On Fri, Jun 13, 2014 at 04:35:01PM -0700, Dave Hansen wrote:
> On 06/13/2014 03:45 PM, Paul E. McKenney wrote:
> > On Fri, Jun 13, 2014 at 01:04:28PM -0700, Dav
> >> So, I bisected it down to this:
> >>
> >>> commit ac1bea85781e9004da9b3e8a4b097c18492d857c
> >>> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >>> Date:   Sun Mar 16 21:36:25 2014 -0700
> >>>
> >>>     sched,rcu: Make cond_resched() report RCU quiescent states
> >>
> >> Specifically, if I raise RCU_COND_RESCHED_LIM, things get back to their
> >> 3.15 levels.
> >>
> >> 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.
> > 
> > Another possibility is that someone is invoking cond_reched() in an
> > incredibly tight loop.
> 
> open() does at least a couple of allocations in getname(),
> get_empty_filp() and apparmor_file_alloc_security() in my kernel, and
> each of those does a cond_resched() via the might_sleep() in the slub
> code.  This test is doing ~400k open/closes per second per CPU, so
> that's ~1.2M cond_resched()/sec/CPU, but that's still hundreds of ns
> between calls on average.
> 
> I'll do some more ftraces and dig in to those debugfs files early next week.
> 
> > But please feel free to send along your patch, CCing LKML.  Longer
> > term, I probably need to take a more algorithmic approach, but what
> > you have will be useful to benchmarkers until then.
> 
> With the caveat that I exerted approximately 15 seconds of brainpower to
> code it up...patch attached.

Thank you Dave!  And if someone doesn't like it, they can always improve
upon it, right?  ;-)

							Thanx, Paul

> ---
> 
>  b/arch/x86/kernel/nmi.c    |    3 +++
>  b/include/linux/rcupdate.h |    2 +-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff -puN arch/x86/kernel/nmi.c~dirty-rcu-hack arch/x86/kernel/nmi.c
> --- a/arch/x86/kernel/nmi.c~dirty-rcu-hack	2014-06-13 16:00:30.257183228 -0700
> +++ b/arch/x86/kernel/nmi.c	2014-06-13 16:00:30.261183407 -0700
> @@ -88,10 +88,13 @@ __setup("unknown_nmi_panic", setup_unkno
> 
>  static u64 nmi_longest_ns = 1 * NSEC_PER_MSEC;
> 
> +u64 RCU_COND_RESCHED_LIM = 256;
>  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);
>  	return 0;
>  }
>  fs_initcall(nmi_warning_debugfs);
> diff -puN include/linux/rcupdate.h~dirty-rcu-hack include/linux/rcupdate.h
> --- a/include/linux/rcupdate.h~dirty-rcu-hack	2014-06-13 16:00:35.578421426 -0700
> +++ b/include/linux/rcupdate.h	2014-06-13 16:00:49.863060683 -0700
> @@ -303,7 +303,7 @@ bool __rcu_is_watching(void);
>   * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
>   */
> 
> -#define RCU_COND_RESCHED_LIM 256	/* ms vs. 100s of ms. */
> +extern u64 RCU_COND_RESCHED_LIM	/* ms vs. 100s of ms. */
>  DECLARE_PER_CPU(int, rcu_cond_resched_count);
>  void rcu_resched(void);
> 
> _

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