Messages in this thread Patch in this message | | | Date | Fri, 13 Jun 2014 16:35:01 -0700 | From | Dave Hansen <> | Subject | Re: [bisected] pre-3.16 regression on open() scalability |
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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@linux.vnet.ibm.com> >>> 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.
---
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); _ | |