mutex: prevent optimistic spinning from spinning longer than neccessary

There is a scalability issue for current implementation of optimistic mutex spin in the kernel. It is found on a 8 node 64 core Nehalem-EX system (HT mode).

The intention of the optimistic mutex spin is to busy wait and spin on a mutex if the owner of the mutex is running, in the hope that the mutex will get released soon and can then be acquired without going to sleep. However, when we have a large number of threads, contending for the mutex, we could have the mutex grabbed by other thread, and then another ……, and we will still be spinning, wasting cpu cycles and adding to the contention. One possible fix is to quit spinning and put the current thread on wait-list if mutex lock switch to a new owner while we spin, indicating heavy contention (see the patch included).

I did some testing on a 8 node Nehalem-EX system with a total of 64 cores. Using Ingo's test-mutex program that creates/delete files with 256 threads (http://lkml.org/lkml/2006/1/8/50) , I see the following speed up after putting in the mutex spin fix:

./mutex-test V 256 10
Ops/sec
2.6.34 62864
With fix 197200

Repeating the test with Aim7 fserver workload, again there is a speed up with the fix:

Jobs/min
2.6.34 91657
With fix 149325

To look at the impact on the distribution of mutex acquisition time, I collected the mutex acquisition time on Aim7 fserver workload with some instrumentation. The average acquisition time is reduced by 48% and number of contentions reduced by 32%.

#contentions Time to acquire mutex (cycles)
2.6.34 72973 44765791
With fix 49210 23067129

The histogram of mutex acquisition time is listed below. The acquisition time is in 2^bin cycles. We see that without the fix, the acquisition time is mostly around 2^26 cycles. With the fix, we the distribution get spread out a lot more towards the lower cycles, starting from 2^13. However, there is an increase of the tail distribution with the fix at 2^28 and 2^29 cycles. I think it is a small price to pay for the reduced average acquisition time and also getting the cpu to do useful work.

Mutex acquisition time distribution (acq time = 2^bin cycles):

	2.6.34			With Fix
bin     #occurrence     %       #occurrence     %
11      2       	0.00%   120     	0.24%
12      10      	0.01%   790     	1.61%
13      14      	0.02%   2058    	4.18%
14      86      	0.12%   3378    	6.86%
15      393     	0.54%   4831    	9.82%
16      710     	0.97%   4893    	9.94%
17      815     	1.12%   4667    	9.48%
18      790     	1.08%   5147    	10.46%
19      580     	0.80%   6250    	12.70%
20      429     	0.59%   6870    	13.96%
21      311     	0.43%   1809    	3.68%
22      255     	0.35%   2305    	4.68%
23      317     	0.44%   916     	1.86%
24      610     	0.84%   233     	0.47%
25      3128    	4.29%   95      	0.19%
26      63902   	87.69%  122     	0.25%
27      619     	0.85%   286     	0.58%
28      0       	0.00%   3536    	7.19%
29      0       	0.00%   903     	1.83%

30 0 0.00% 0 0.00%

Regards,
Tim

diff -ur linux-2.6.34/kernel/sched.c linux-2.6.34-fix/kernel/sched.c
@@ -3815,8 +3815,11 @@
 		/*
 		 * Owner changed, break to re-assess state.
 		 */
-		if (lock->owner != owner)
+		if (lock->owner != owner) {
+			if (lock->owner)
+				return 0;
 			break;
+		}
 
 		/*
 		 * Is that owner really running on that cpu?