md/raid6 algorithms: delta syndrome functions

v3: s-o-b comment, explanation of performance and descision for
the start/stop implementation

Implementing rmw functionality for RAID6 requires optimized syndrome
calculation. Up to now we can only generate a complete syndrome. The
target P/Q pages are always overwritten. With this patch we provide
a framework for inplace P/Q modification. In the first place simply
fill those functions with NULL values.

xor_syndrome() has two additional parameters: start & stop. These
will indicate the first and last page that are changing during a
rmw run. That makes it possible to avoid several unneccessary loops
and speed up calculation. The caller needs to implement the following
logic to make the functions work.

1) xor_syndrome(disks, start, stop, ...): "Remove" all data of source
blocks inside P/Q between (and including) start and end.

2) modify any block with start <= block <= stop

3) xor_syndrome(disks, start, stop, ...): "Reinsert" all data of
source blocks into P/Q between (and including) start and end.

Pages between start and stop that won't be changed should be filled
with a pointer to the kernel zero page. The reasons for not taking NULL
pages are:

1) Algorithms cross the whole source data line by line. Thus avoid
additional branches.

2) Having a NULL page avoids calculating the XOR P parity but still
need calulation steps for the Q parity. Depending on the algorithm
unrolling that might be only a difference of 2 instructions per loop.

The benchmark numbers of the gen_syndrome() functions are displayed in
the kernel log. Do the same for the xor_syndrome() functions. This
will help to analyze performance problems and give an rough estimate
how well the algorithm works. The choice of the fastest algorithm will
still depend on the gen_syndrome() performance.

With the start/stop page implementation the speed can vary a lot in real
life. E.g. a change of page 0 & page 15 on a stripe will be harder to
compute than the case where page 0 & page 1 are XOR candidates. To be not
to enthusiatic about the expected speeds we will run a worse case test
that simulates a change on the upper half of the stripe. So we do:

1) calculation of P/Q for the upper pages

2) continuation of Q for the lower (empty) pages

Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: NeilBrown <neilb@suse.de>

include/linux/raid/pq.h

@@ -72,6 +72,7 @@ extern const char raid6_empty_zero_page[PAGE_SIZE];
 /* Routine choices */
 struct raid6_calls {
 	void (*gen_syndrome)(int, size_t, void **);
+	void (*xor_syndrome)(int, int, int, size_t, void **);
 	int  (*valid)(void);	/* Returns 1 if this routine set is usable */
 	const char *name;	/* Name of this routine set */
 	int prefer;		/* Has special performance attribute */

lib/raid6/algos.c

@@ -131,11 +131,12 @@ static inline const struct raid6_recov_calls *raid6_choose_recov(void)
 static inline const struct raid6_calls *raid6_choose_gen(
 	void *(*const dptrs)[(65536/PAGE_SIZE)+2], const int disks)
 {
-	unsigned long perf, bestperf, j0, j1;
+	unsigned long perf, bestgenperf, bestxorperf, j0, j1;
+	int start = (disks>>1)-1, stop = disks-3;	/* work on the second half of the disks */
 	const struct raid6_calls *const *algo;
 	const struct raid6_calls *best;
 
-	for (bestperf = 0, best = NULL, algo = raid6_algos; *algo; algo++) {
+	for (bestgenperf = 0, bestxorperf = 0, best = NULL, algo = raid6_algos; *algo; algo++) {
 		if (!best || (*algo)->prefer >= best->prefer) {
 			if ((*algo)->valid && !(*algo)->valid())
 				continue;
@@ -153,19 +154,45 @@ static inline const struct raid6_calls *raid6_choose_gen(
 			}
 			preempt_enable();
 
-			if (perf > bestperf) {
-				bestperf = perf;
+			if (perf > bestgenperf) {
+				bestgenperf = perf;
 				best = *algo;
 			}
-			pr_info("raid6: %-8s %5ld MB/s\n", (*algo)->name,
+			pr_info("raid6: %-8s gen() %5ld MB/s\n", (*algo)->name,
 			       (perf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
+
+			if (!(*algo)->xor_syndrome)
+				continue;
+
+			perf = 0;
+
+			preempt_disable();
+			j0 = jiffies;
+			while ((j1 = jiffies) == j0)
+				cpu_relax();
+			while (time_before(jiffies,
+					    j1 + (1<<RAID6_TIME_JIFFIES_LG2))) {
+				(*algo)->xor_syndrome(disks, start, stop,
+						      PAGE_SIZE, *dptrs);
+				perf++;
+			}
+			preempt_enable();
+
+			if (best == *algo)
+				bestxorperf = perf;
+
+			pr_info("raid6: %-8s xor() %5ld MB/s\n", (*algo)->name,
+				(perf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2+1));
 		}
 	}
 
 	if (best) {
-		pr_info("raid6: using algorithm %s (%ld MB/s)\n",
+		pr_info("raid6: using algorithm %s gen() %ld MB/s\n",
 		       best->name,
-		       (bestperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
+		       (bestgenperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
+		if (best->xor_syndrome)
+			pr_info("raid6: .... xor() %ld MB/s, rmw enabled\n",
+			       (bestxorperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2+1));
 		raid6_call = *best;
 	} else
 		pr_err("raid6: Yikes!  No algorithm found!\n");

lib/raid6/altivec.uc

@@ -119,6 +119,7 @@ int raid6_have_altivec(void)
 
 const struct raid6_calls raid6_altivec$# = {
 	raid6_altivec$#_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_altivec,
 	"altivecx$#",
 	0

lib/raid6/avx2.c

@@ -89,6 +89,7 @@ static void raid6_avx21_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_avx2x1 = {
 	raid6_avx21_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_avx2,
 	"avx2x1",
 	1			/* Has cache hints */
@@ -150,6 +151,7 @@ static void raid6_avx22_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_avx2x2 = {
 	raid6_avx22_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_avx2,
 	"avx2x2",
 	1			/* Has cache hints */
@@ -242,6 +244,7 @@ static void raid6_avx24_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_avx2x4 = {
 	raid6_avx24_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_avx2,
 	"avx2x4",
 	1			/* Has cache hints */

lib/raid6/int.uc

@@ -109,7 +109,8 @@ static void raid6_int$#_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_intx$# = {
 	raid6_int$#_gen_syndrome,
-	NULL,		/* always valid */
+	NULL,			/* XOR not yet implemented */
+	NULL,			/* always valid */
 	"int" NSTRING "x$#",
 	0
 };

lib/raid6/mmx.c

@@ -76,6 +76,7 @@ static void raid6_mmx1_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_mmxx1 = {
 	raid6_mmx1_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_mmx,
 	"mmxx1",
 	0
@@ -134,6 +135,7 @@ static void raid6_mmx2_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_mmxx2 = {
 	raid6_mmx2_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_mmx,
 	"mmxx2",
 	0

lib/raid6/neon.c

@@ -42,6 +42,7 @@
 	}								\
 	struct raid6_calls const raid6_neonx ## _n = {			\
 		raid6_neon ## _n ## _gen_syndrome,			\
+		NULL,		/* XOR not yet implemented */		\
 		raid6_have_neon,					\
 		"neonx" #_n,						\
 		0							\

lib/raid6/sse1.c

@@ -92,6 +92,7 @@ static void raid6_sse11_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_sse1x1 = {
 	raid6_sse11_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_sse1_or_mmxext,
 	"sse1x1",
 	1			/* Has cache hints */
@@ -154,6 +155,7 @@ static void raid6_sse12_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_sse1x2 = {
 	raid6_sse12_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_sse1_or_mmxext,
 	"sse1x2",
 	1			/* Has cache hints */

lib/raid6/sse2.c

@@ -90,6 +90,7 @@ static void raid6_sse21_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_sse2x1 = {
 	raid6_sse21_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_sse2,
 	"sse2x1",
 	1			/* Has cache hints */
@@ -152,6 +153,7 @@ static void raid6_sse22_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_sse2x2 = {
 	raid6_sse22_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_sse2,
 	"sse2x2",
 	1			/* Has cache hints */
@@ -250,6 +252,7 @@ static void raid6_sse24_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_sse2x4 = {
 	raid6_sse24_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	raid6_have_sse2,
 	"sse2x4",
 	1			/* Has cache hints */

lib/raid6/tilegx.uc

@@ -80,6 +80,7 @@ void raid6_tilegx$#_gen_syndrome(int disks, size_t bytes, void **ptrs)
 
 const struct raid6_calls raid6_tilegx$# = {
 	raid6_tilegx$#_gen_syndrome,
+	NULL,			/* XOR not yet implemented */
 	NULL,
 	"tilegx$#",
 	0