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>

diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index 73069cb6c54a15ea01bacdd9af20170be6ba257a..a7a06d1dcf9cb17ee65d1befe1ef6ac62d882d16 100644 (file)
--- a/include/linux/raid/pq.h
+++ b/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 */

diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index dbef2314901ed39c54b61d9511e0dbce37ddaa83..975c6e0434bde0b4b804654822928899ac9e9e2a 100644 (file)
--- a/lib/raid6/algos.c
+++ b/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");

diff --git a/lib/raid6/altivec.uc b/lib/raid6/altivec.uc
index 7cc12b532e95b90d85e3968099359a8ba19b2708..bec27fce7501702a7da2a16c621c1b91adb938f5 100644 (file)
--- a/lib/raid6/altivec.uc
+++ b/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

diff --git a/lib/raid6/avx2.c b/lib/raid6/avx2.c
index bc3b1dd436eb4d4d807e08569237424032e0adf4..76734004358da609f6062f1095cdae89fb038b7e 100644 (file)
--- a/lib/raid6/avx2.c
+++ b/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 */

diff --git a/lib/raid6/int.uc b/lib/raid6/int.uc
index 5b50f8dfc5d244efa4678e1034da785b296559fd..5ca60bee138879a2b9bd12f848f52fcdd297f261 100644 (file)
--- a/lib/raid6/int.uc
+++ b/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
 };

diff --git a/lib/raid6/mmx.c b/lib/raid6/mmx.c
index 590c71c9e2005b407166cde2e37a0c8066c7ef0d..b3b0e1fcd3af8b6d13f72b1501275f472c4f06ec 100644 (file)
--- a/lib/raid6/mmx.c
+++ b/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

diff --git a/lib/raid6/neon.c b/lib/raid6/neon.c
index 36ad4705df1aef883d32e79d8e41b92569741a07..d9ad6ee284f4b0ae33961fe42528de2df69b591b 100644 (file)
--- a/lib/raid6/neon.c
+++ b/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                                                       \

diff --git a/lib/raid6/sse1.c b/lib/raid6/sse1.c
index f7629713944505e575e2d2746c62ebaef3ca6946..9025b8ca9aa3b5968faa31d2a8d0e27e3abd56bc 100644 (file)
--- a/lib/raid6/sse1.c
+++ b/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 */

diff --git a/lib/raid6/sse2.c b/lib/raid6/sse2.c
index 85b82c85f28e61699bbf94e19e3f2f7750108d10..31acd59a0ef7ef92ec960ed64fffa233615ee973 100644 (file)
--- a/lib/raid6/sse2.c
+++ b/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 */

diff --git a/lib/raid6/tilegx.uc b/lib/raid6/tilegx.uc
index e7c29459cbcd5a69fc929f1994cdeab02bea6148..2dd291a112645f82450cca76431e5d72e7884c5d 100644 (file)
--- a/lib/raid6/tilegx.uc
+++ b/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