lkml.org 
[lkml]   [2009]   [Jun]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[Patch v2] stop_machine stalls for a considerable period on large cpu count machines.
I forgot again on the repost.

Sorry for the noise,
Robin

----- Forwarded message from Robin Holt <holt@sgi.com> -----

Date: Sat, 27 Jun 2009 06:34:10 -0500
From: Robin Holt <holt@sgi.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Travis <travis@sgi.com>, Rusty Russell <rusty@rustcorp.com.au>,
Stable Kernel Maintainers <stable@kernel.org>
Subject: [Patch v2] stop_machine stalls for a considerable period on large
cpu count machines.

Mike Travis noted that a 2048 cpu machine booting would take hours
to get through its modprobes. We would get numerous back traces from
stop_cpu indicating they had not serviced interrupts.

A quick code review indicated we have a situation of heavy cacheline
contention due to the 'state' (read-mostly) and 'thread_ack'
(write-mostly) variables being located in the same cacheline.


Signed-off-by: Robin Holt <holt@sgi.com>
Cc: Mike Travis <travis@sgi.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Stable Kernel Maintainers <stable@kernel.org>
---

My first attempt missed a 'quilt refresh' and did not work.

kernel/stop_machine.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)


Index: stop_machine_false_sharing/kernel/stop_machine.c
===================================================================
--- stop_machine_false_sharing.orig/kernel/stop_machine.c 2009-06-27 06:30:24.196637521 -0500
+++ stop_machine_false_sharing/kernel/stop_machine.c 2009-06-27 06:30:28.401164425 -0500
@@ -13,6 +13,13 @@
#include <asm/atomic.h>
#include <asm/uaccess.h>

+/*
+ * It is important to keep 'thread_ack' and 'state' in a seperate
+ * cachelines to prevent cacheline sharing between threads updating
+ * thread_ack and other threads spinning on state.
+ */
+static atomic_t thread_ack ____cacheline_aligned;
+
/* This controls the threads on each CPU. */
enum stopmachine_state {
/* Dummy starting state for thread. */
@@ -26,7 +33,7 @@ enum stopmachine_state {
/* Exit */
STOPMACHINE_EXIT,
};
-static enum stopmachine_state state;
+static enum stopmachine_state state ____cacheline_aligned;

struct stop_machine_data {
int (*fn)(void *);
@@ -36,7 +43,6 @@ struct stop_machine_data {

/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
static unsigned int num_threads;
-static atomic_t thread_ack;
static DEFINE_MUTEX(lock);
/* setup_lock protects refcount, stop_machine_wq and stop_machine_work. */
static DEFINE_MUTEX(setup_lock);
----- End forwarded message -----

\
 
 \ /
  Last update: 2009-06-27 13:43    [W:0.029 / U:0.412 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site