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>

1 files changed, 34 insertions, 7 deletions

diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index dbef2314901e..975c6e0434bd 100644
--- 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");