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Subject[Patch] stop_machine stalls for a considerable period on large cpu count machines.
I forgot to include the linux-kernel mailing list.

Robin

----- Forwarded message from Robin Holt <holt@sgi.com> -----
Date: Sat, 27 Jun 2009 05:53:11 -0500
From: Robin Holt <holt@sgi.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>, stable@kernel.org,
mike travis <travis@sgi.com>
Subject: [Patch] 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>

---
kernel/stop_machine.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
Index: stop_machine_false_sharing/kernel/stop_machine.c
===================================================================
--- stop_machine_false_sharing.orig/kernel/stop_machine.c 2009-06-27 05:26:57.000000000 -0500
+++ stop_machine_false_sharing/kernel/stop_machine.c 2009-06-27 05:34:45.182637682 -0500
@@ -36,8 +36,8 @@ 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);
+static atomic_t thread_ack ____cacheline_aligned;
+static DEFINE_MUTEX(lock) ____cacheline_aligned;
/* setup_lock protects refcount, stop_machine_wq and stop_machine_work. */
static DEFINE_MUTEX(setup_lock);
/* Users of stop_machine. */
----- End forwarded message -----

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