Messages in this thread Patch in this message | | | Date | Sat, 27 Jun 2009 06:40:29 -0500 | From | Robin Holt <> | Subject | [Patch v2] stop_machine stalls for a considerable period on large cpu count machines. |
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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 -----
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