linux/lib/raid6/neon.c
Markus Stockhausen fe5cbc6e06 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>
2015-04-22 08:00:41 +10:00

60 lines
1.7 KiB
C

/*
* linux/lib/raid6/neon.c - RAID6 syndrome calculation using ARM NEON intrinsics
*
* Copyright (C) 2013 Linaro Ltd <ard.biesheuvel@linaro.org>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/raid/pq.h>
#ifdef __KERNEL__
#include <asm/neon.h>
#else
#define kernel_neon_begin()
#define kernel_neon_end()
#define cpu_has_neon() (1)
#endif
/*
* There are 2 reasons these wrappers are kept in a separate compilation unit
* from the actual implementations in neonN.c (generated from neon.uc by
* unroll.awk):
* - the actual implementations use NEON intrinsics, and the GCC support header
* (arm_neon.h) is not fully compatible (type wise) with the kernel;
* - the neonN.c files are compiled with -mfpu=neon and optimization enabled,
* and we have to make sure that we never use *any* NEON/VFP instructions
* outside a kernel_neon_begin()/kernel_neon_end() pair.
*/
#define RAID6_NEON_WRAPPER(_n) \
static void raid6_neon ## _n ## _gen_syndrome(int disks, \
size_t bytes, void **ptrs) \
{ \
void raid6_neon ## _n ## _gen_syndrome_real(int, \
unsigned long, void**); \
kernel_neon_begin(); \
raid6_neon ## _n ## _gen_syndrome_real(disks, \
(unsigned long)bytes, ptrs); \
kernel_neon_end(); \
} \
struct raid6_calls const raid6_neonx ## _n = { \
raid6_neon ## _n ## _gen_syndrome, \
NULL, /* XOR not yet implemented */ \
raid6_have_neon, \
"neonx" #_n, \
0 \
}
static int raid6_have_neon(void)
{
return cpu_has_neon();
}
RAID6_NEON_WRAPPER(1);
RAID6_NEON_WRAPPER(2);
RAID6_NEON_WRAPPER(4);
RAID6_NEON_WRAPPER(8);