On Mon, Feb 02, 2015 at 03:29:09PM -0800, Andrew Morton wrote:

On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin" [off-list ref] wrote:

quoted

This patch change non-atomic bitops,
add a if() condition to test it, before set/clear the bit.
so that we don't need dirty the cache line, if this bit
have been set or clear. On SMP system, dirty cache line will
need invalidate other processors cache line, this will have
some impact on SMP systems.

--- a/include/asm-generic/bitops/non-atomic.h
+++ b/include/asm-generic/bitops/non-atomic.h

@@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile unsigned long *addr)
 	unsigned long mask = BIT_MASK(nr);
 	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
 
-	*p  |= mask;
+	if ((*p & mask) == 0)
+		*p  |= mask;
+
 }

hm, maybe.

It will speed up set_bit on an already-set bit.  But it will slow down
set_bit on a not-set bit.  And the latter case is presumably much, much
more common.

How do we know the patch is a net performance gain?

Let's try to measure. The micro benchmark:

	#include <stdio.h>
	#include <time.h>
	#include <sys/mman.h>

	#ifdef CACHE_HOT
	#define SIZE (2UL << 20)
	#define TIMES 10000000
	#else
	#define SIZE (1UL << 30)
	#define TIMES 10000
	#endif

	int main(int argc, char **argv)
	{
		struct timespec a, b, diff;
		unsigned long i, *p, times = TIMES;

		p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
				MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1, 0);
		
		clock_gettime(CLOCK_MONOTONIC, &a);
		while (times--) {
			for (i = 0; i < SIZE/64/sizeof(*p); i++) {
	#ifdef CHECK_BEFORE_SET
				if (p[i] != times)
	#endif
					p[i] = times;
			}
		}
		clock_gettime(CLOCK_MONOTONIC, &b);

		diff.tv_sec = b.tv_sec - a.tv_sec;
		if (a.tv_nsec > b.tv_nsec) {
			diff.tv_sec--;
			diff.tv_nsec = 1000000000 + b.tv_nsec - a.tv_nsec;
		} else
			diff.tv_nsec = b.tv_nsec - a.tv_nsec;

		printf("%lu.%09lu\n", diff.tv_sec, diff.tv_nsec);
		return 0;
	}

Results for 10 runs on my laptop -- i5-3427U (IvyBridge 1.8 Ghz, 2.8Ghz Turbo
with 3MB LLC):

				Avg		Stddev
baseline			21.5351		0.5315
-DCHECK_BEFORE_SET		21.9834		0.0789
-DCACHE_HOT			14.9987		0.0365
-DCACHE_HOT -DCHECK_BEFORE_SET	29.9010		0.0204

Difference between -DCACHE_HOT and -DCACHE_HOT -DCHECK_BEFORE_SET appears
huge, but if you recalculate it to CPU cycles per inner loop @ 2.8 Ghz,
it's 1.02530 and 2.04401 CPU cycles respectively.

Basically, the check is free on decent CPU. 

-- 
 Kirill A. Shutemov