[PATCH v2] lib: optimize cpumask_next_and()

[Clement Courbet]

We've measured that we spend ~0.6% of sys cpu time in cpumask_next_and().
It's essentially a joined iteration in search for a non-zero bit, which
is currently implemented as a lookup join (find a nonzero bit on the
lhs, lookup the rhs to see if it's set there).

Implement a direct join (find a nonzero bit on the incrementally built
join). Direct benchmarking shows that it's 1.17x to 14x faster with a
geometric mean of 2.1 on 32 CPUs. No impact on memory usage.

Approximate benchmark code:

```
  unsigned long src1p[nr_cpumask_longs] = {pattern1};
  unsigned long src2p[nr_cpumask_longs] = {pattern2};
  for (/*a bunch of repetitions*/) {
    for (int n = -1; n <= nr_cpu_ids; ++n) {
      asm volatile("" : "+rm"(src1p)); // prevent any optimization
      asm volatile("" : "+rm"(src2p));
      unsigned long result = cpumask_next_and(n, src1p, src2p);
      asm volatile("" : "+rm"(result));
    }
  }
```

Results:
pattern1    pattern2     time_before/time_after
0x0000ffff  0x0000ffff   1.65
0x0000ffff  0x00005555   2.24
0x0000ffff  0x00001111   2.94
0x0000ffff  0x00000000   14.0
0x00005555  0x0000ffff   1.67
0x00005555  0x00005555   1.71
0x00005555  0x00001111   1.90
0x00005555  0x00000000   6.58
0x00001111  0x0000ffff   1.46
0x00001111  0x00005555   1.49
0x00001111  0x00001111   1.45
0x00001111  0x00000000   3.10
0x00000000  0x0000ffff   1.18
0x00000000  0x00005555   1.18
0x00000000  0x00001111   1.17
0x00000000  0x00000000   1.25
-----------------------------
               geo.mean  2.06

Signed-off-by: Clement Courbet <courbet@xxxxxxxxxx>
---
Changes in v2:
 - Refactored _find_next_common_bit into _find_next_bit., as suggested
   by Yury Norov. This has no adverse effects on the performance side,
   as the compiler successfully inlines the code.

 include/asm-generic/bitops/find.h       | 16 ++++++++++++++
 include/linux/bitmap.h                  |  2 ++
 lib/cpumask.c                           |  9 ++++----
 lib/find_bit.c                          | 37 +++++++++++++++++++++++++--------
 tools/include/asm-generic/bitops/find.h | 16 ++++++++++++++
 5 files changed, 67 insertions(+), 13 deletions(-)

diff --git a/include/asm-generic/bitops/find.h b/include/asm-generic/bitops/find.h
index 998d4d544f18..130962f3a264 100644
--- a/include/asm-generic/bitops/find.h
+++ b/include/asm-generic/bitops/find.h
@@ -15,6 +15,22 @@ extern unsigned long find_next_bit(const unsigned long *addr, unsigned long
 		size, unsigned long offset);
 #endif
 
+#ifndef find_next_and_bit
+/**
+ * find_next_and_bit - find the next set bit in both memory regions
+ * @addr1: The first address to base the search on
+ * @addr2: The second address to base the search on
+ * @offset: The bitnumber to start searching at
+ * @size: The bitmap size in bits
+ *
+ * Returns the bit number for the next set bit
+ * If no bits are set, returns @size.
+ */
+extern unsigned long find_next_and_bit(const unsigned long *addr1,
+		const unsigned long *addr2, unsigned long size,
+		unsigned long offset);
+#endif
+
 #ifndef find_next_zero_bit
 /**
  * find_next_zero_bit - find the next cleared bit in a memory region
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 700cf5f67118..b4606bfda52f 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -77,6 +77,8 @@
  * find_first_bit(addr, nbits)		Position first set bit in *addr
  * find_next_zero_bit(addr, nbits, bit)	Position next zero bit in *addr >= bit
  * find_next_bit(addr, nbits, bit)	Position next set bit in *addr >= bit
+ * find_next_and_bit(addr1, addr2, nbits, bit)	Same as find_first_bit, but in
+ *						(*addr1 & *addr2)
  */
 
 /*
diff --git a/lib/cpumask.c b/lib/cpumask.c
index 8b1a1bd77539..5602223837fa 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -32,10 +32,11 @@ EXPORT_SYMBOL(cpumask_next);
 int cpumask_next_and(int n, const struct cpumask *src1p,
 		     const struct cpumask *src2p)
 {
-	while ((n = cpumask_next(n, src1p)) < nr_cpu_ids)
-		if (cpumask_test_cpu(n, src2p))
-			break;
-	return n;
+	/* -1 is a legal arg here. */
+	if (n != -1)
+		cpumask_check(n);
+	return find_next_and_bit(cpumask_bits(src1p), cpumask_bits(src2p),
+		nr_cpumask_bits, n + 1);
 }
 EXPORT_SYMBOL(cpumask_next_and);
 
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 6ed74f78380c..ebc08fd9fdf8 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -24,19 +24,25 @@
 #if !defined(find_next_bit) || !defined(find_next_zero_bit)
 
 /*
- * This is a common helper function for find_next_bit and
- * find_next_zero_bit.  The difference is the "invert" argument, which
- * is XORed with each fetched word before searching it for one bits.
+ * This is a common helper function for find_next_bit, find_next_zero_bit, and
+ * find_next_and_bit. The differences are:
+ *  - The "invert" argument, which is XORed with each fetched word before
+ *    searching it for one bits.
+ *  - The optional "addr2", which is anded with "addr1" if present.
  */
-static unsigned long _find_next_bit(const unsigned long *addr,
-		unsigned long nbits, unsigned long start, unsigned long invert)
+static unsigned long _find_next_bit(const unsigned long *addr1,
+		const unsigned long *addr2, unsigned long nbits,
+		unsigned long start, unsigned long invert)
 {
 	unsigned long tmp;
 
 	if (unlikely(start >= nbits))
 		return nbits;
 
-	tmp = addr[start / BITS_PER_LONG] ^ invert;
+	tmp = addr1[start / BITS_PER_LONG];
+	if (addr2)
+		tmp &= addr2[start / BITS_PER_LONG];
+	tmp ^= invert;
 
 	/* Handle 1st word. */
 	tmp &= BITMAP_FIRST_WORD_MASK(start);
@@ -47,7 +53,10 @@ static unsigned long _find_next_bit(const unsigned long *addr,
 		if (start >= nbits)
 			return nbits;
 
-		tmp = addr[start / BITS_PER_LONG] ^ invert;
+		tmp = addr1[start / BITS_PER_LONG];
+		if (addr2)
+			tmp &= addr2[start / BITS_PER_LONG];
+		tmp ^= invert;
 	}
 
 	return min(start + __ffs(tmp), nbits);
@@ -61,7 +70,7 @@ static unsigned long _find_next_bit(const unsigned long *addr,
 unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
 			    unsigned long offset)
 {
-	return _find_next_bit(addr, size, offset, 0UL);
+	return _find_next_bit(addr, NULL, size, offset, 0UL);
 }
 EXPORT_SYMBOL(find_next_bit);
 #endif
@@ -70,11 +79,21 @@ EXPORT_SYMBOL(find_next_bit);
 unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
 				 unsigned long offset)
 {
-	return _find_next_bit(addr, size, offset, ~0UL);
+	return _find_next_bit(addr, NULL, size, offset, ~0UL);
 }
 EXPORT_SYMBOL(find_next_zero_bit);
 #endif
 
+#if !defined(find_next_and_bit)
+unsigned long find_next_and_bit(const unsigned long *addr1,
+		const unsigned long *addr2, unsigned long nbits,
+		unsigned long start)
+{
+	return _find_next_bit(addr1, addr2, size, offset, ~0UL);
+}
+EXPORT_SYMBOL(find_next_and_bit);
+#endif
+
 #ifndef find_first_bit
 /*
  * Find the first set bit in a memory region.
diff --git a/tools/include/asm-generic/bitops/find.h b/tools/include/asm-generic/bitops/find.h
index 5538ecdc964a..435c7d002f33 100644
--- a/tools/include/asm-generic/bitops/find.h
+++ b/tools/include/asm-generic/bitops/find.h
@@ -15,6 +15,22 @@ extern unsigned long find_next_bit(const unsigned long *addr, unsigned long
 		size, unsigned long offset);
 #endif
 
+#ifndef find_next_and_bit
+/**
+ * find_next_and_bit - find the next set bit in both memory regions
+ * @addr1: The first address to base the search on
+ * @addr2: The second address to base the search on
+ * @offset: The bitnumber to start searching at
+ * @size: The bitmap size in bits
+ *
+ * Returns the bit number for the next set bit
+ * If no bits are set, returns @size.
+ */
+extern unsigned long find_next_and_bit(const unsigned long *addr1,
+		const unsigned long *addr2, unsigned long size,
+		unsigned long offset);
+#endif
+
 #ifndef find_next_zero_bit
 
 /**
-- 
2.15.0.rc0.271.g36b669edcc-goog